WooCommerce Upsell and Cross-Sell: Complete Guide (woosellservices.com)

01-set-upsell-products.php

<?php
/**
 * Programmatically set upsell products for a WooCommerce product.
 *
 * WooCommerce stores upsell product IDs in post meta. This
 * function demonstrates how to set upsells via code, which
 * is useful when migrating data, syncing from an ERP, or
 * building automated product recommendation logic.
 *
 * @param int   $product_id   The product to add upsells to.
 * @param array $upsell_ids   Array of product IDs to upsell.
 */
function woosell_set_upsell_products( $product_id, $upsell_ids ) {
    // Load the WooCommerce product object
    $product = wc_get_product( $product_id );

    if ( ! $product ) {
        return new WP_Error( 'invalid_product', 'Product not found: ' . $product_id );
    }

    // Validate that all upsell IDs are real published products
    $valid_ids = array();
    foreach ( $upsell_ids as $id ) {
        $upsell_product = wc_get_product( $id );
        if ( $upsell_product && 'publish' === $upsell_product->get_status() ) {
            $valid_ids[] = $id;
        }
    }

    // Set the upsell IDs and save the product
    // WooCommerce displays these on the single product page
    $product->set_upsell_ids( $valid_ids );
    $product->save();

    return count( $valid_ids ) . ' upsell products set for ' . $product->get_name();
}

02-cross-sell-products.php

<?php
/**
 * Customize WooCommerce cross-sell display on the cart page.
 *
 * By default, WooCommerce shows cross-sells below the cart
 * table. This function controls how many cross-sells appear
 * and adds custom ordering logic to prioritize products with
 * the highest conversion potential (based on sales data).
 *
 * @param array $cross_sells Array of cross-sell product IDs from cart items.
 * @return array Filtered and prioritized cross-sell product IDs.
 */
function woosell_prioritize_cross_sells( $cross_sells ) {
    if ( empty( $cross_sells ) ) {
        return $cross_sells;
    }

    // Score each cross-sell product by total sales count
    // Products that sell well are more likely to convert
    $scored = array();
    foreach ( $cross_sells as $product_id ) {
        $product = wc_get_product( $product_id );
        if ( $product ) {
            $scored[ $product_id ] = $product->get_total_sales();
        }
    }

    // Sort by sales descending so best sellers appear first
    arsort( $scored );

    // Return only top 4 cross-sell products to avoid clutter
    return array_slice( array_keys( $scored ), 0, 4 );
}
add_filter( 'woocommerce_cart_crosssell_ids', 'woosell_prioritize_cross_sells' );

/**
 * Change the number of cross-sell columns displayed on cart page.
 * Default is 2 columns; 4 columns works better for visual products.
 */
add_filter( 'woocommerce_cross_sells_columns', function() {
    return 4;
});

03-checkout-order-bump.php

<?php
/**
 * Add a one-click order bump above the Place Order button.
 *
 * Displays a highlighted product offer that customers can
 * add to their cart with a single checkbox click during
 * checkout. This technique typically converts at 5-15%
 * and significantly increases average order value.
 *
 * @since 1.0.0
 */

// Define the bump product ID and discount percentage
define( 'WOOSELL_BUMP_PRODUCT_ID', 123 );
define( 'WOOSELL_BUMP_DISCOUNT', 30 );

/**
 * Display the order bump offer box at checkout.
 * Hooked just before the payment section for maximum visibility.
 */
function woosell_display_order_bump() {
    $product = wc_get_product( WOOSELL_BUMP_PRODUCT_ID );

    if ( ! $product || ! $product->is_purchasable() ) {
        return;
    }

    // Don't show if product is already in cart
    foreach ( WC()->cart->get_cart() as $cart_item ) {
        if ( $cart_item['product_id'] === WOOSELL_BUMP_PRODUCT_ID ) {
            return;
        }
    }

    $regular_price = $product->get_regular_price();
    $bump_price    = $regular_price * ( 1 - WOOSELL_BUMP_DISCOUNT / 100 );

    // Render the order bump HTML with inline styles
    printf(
        '<div id="woosell-order-bump" style="border:2px dashed #f0c14b;background:#fffbf0;padding:20px;margin:20px 0;border-radius:8px;">
            <label style="display:flex;align-items:center;gap:12px;cursor:pointer;font-size:16px;">
                <input type="checkbox" name="woosell_add_bump" value="1" style="width:20px;height:20px;" />
                <span>
                    <strong>One-Time Offer:</strong> Add %s for just %s
                    <span style="text-decoration:line-through;color:#999;">%s</span>
                    <span style="color:#e63946;font-weight:bold;">(%d%% OFF)</span>
                </span>
            </label>
            <p style="margin:8px 0 0 32px;color:#666;font-size:14px;">%s</p>
        </div>',
        esc_html( $product->get_name() ),
        wc_price( $bump_price ),
        wc_price( $regular_price ),
        WOOSELL_BUMP_DISCOUNT,
        esc_html( $product->get_short_description() )
    );
}
add_action( 'woocommerce_review_order_before_payment', 'woosell_display_order_bump' );

/**
 * Process the order bump: add product to cart if checkbox was checked.
 * Applies the discount as a custom fee so it shows in order totals.
 */
function woosell_process_order_bump() {
    if ( ! isset( $_POST['woosell_add_bump'] ) ) {
        return;
    }

    $product = wc_get_product( WOOSELL_BUMP_PRODUCT_ID );
    if ( ! $product ) {
        return;
    }

    // Add to cart and apply discount as negative fee
    WC()->cart->add_to_cart( WOOSELL_BUMP_PRODUCT_ID );
    $discount = $product->get_regular_price() * ( WOOSELL_BUMP_DISCOUNT / 100 );

    WC()->cart->add_fee(
        sprintf( '%s - %d%% checkout discount', $product->get_name(), WOOSELL_BUMP_DISCOUNT ),
        -$discount
    );
}
add_action( 'woocommerce_checkout_update_order_review', 'woosell_process_order_bump' );

04-thank-you-page-upsell.php

<?php
/**
 * Display a post-purchase upsell offer on the thank-you page.
 *
 * After completing a purchase, customers are in a buying
 * mindset. Showing a relevant product offer on the order
 * confirmation page converts at 3-8% according to industry
 * benchmarks. This adds the product directly to a new cart
 * for one-click purchase.
 *
 * @param int $order_id The completed order ID.
 */
function woosell_thankyou_upsell( $order_id ) {
    $order = wc_get_order( $order_id );

    if ( ! $order ) {
        return;
    }

    // Find a relevant upsell based on what was just purchased
    $upsell_id = null;
    foreach ( $order->get_items() as $item ) {
        $product    = $item->get_product();
        $upsell_ids = $product->get_upsell_ids();

        if ( ! empty( $upsell_ids ) ) {
            // Pick the first upsell that wasn't in this order
            $ordered_ids = wp_list_pluck( $order->get_items(), 'product_id' );
            $available   = array_diff( $upsell_ids, $ordered_ids );

            if ( ! empty( $available ) ) {
                $upsell_id = reset( $available );
                break;
            }
        }
    }

    if ( ! $upsell_id ) {
        return;
    }

    $upsell = wc_get_product( $upsell_id );
    if ( ! $upsell || ! $upsell->is_purchasable() ) {
        return;
    }

    // Build the add-to-cart URL for one-click purchase
    $add_url = add_query_arg(
        array(
            'add-to-cart' => $upsell_id,
            'quantity'    => 1,
        ),
        wc_get_checkout_url()
    );

    // Display the upsell offer
    printf(
        '<div style="background:#f8f9fa;border:1px solid #dee2e6;border-radius:12px;padding:24px;margin:24px 0;text-align:center;">
            <h3 style="margin-top:0;">Complete Your Purchase</h3>
            <p>Customers who bought %s also love:</p>
            <div style="margin:16px 0;">%s</div>
            <h4>%s</h4>
            <p style="font-size:24px;font-weight:bold;color:#2d3436;">%s</p>
            <a href="%s" style="display:inline-block;background:#0073aa;color:#fff;padding:12px 32px;border-radius:6px;text-decoration:none;font-weight:bold;">Add to Cart &amp; Checkout</a>
        </div>',
        esc_html( $order->get_items() ? current( $order->get_items() )->get_name() : '' ),
        $upsell->get_image( 'woocommerce_thumbnail' ),
        esc_html( $upsell->get_name() ),
        $upsell->get_price_html(),
        esc_url( $add_url )
    );
}
add_action( 'woocommerce_thankyou', 'woosell_thankyou_upsell', 5 );

05-smart-related-products.php

<?php
/**
 * Override WooCommerce's default related products logic.
 *
 * The default algorithm uses categories and tags but is
 * not always optimal for conversions. This replacement
 * prioritizes products from the same category that have
 * higher sales counts, then fills remaining slots with
 * products from related categories.
 *
 * @param array $related_posts Default related product IDs from WooCommerce.
 * @param int   $product_id   The current product being viewed.
 * @param array $args          Query arguments including limit and excluded IDs.
 * @return array Modified array of related product IDs.
 */
function woosell_smart_related_products( $related_posts, $product_id, $args ) {
    // Get the primary category of the current product
    $categories = wp_get_post_terms( $product_id, 'product_cat', array( 'fields' => 'ids' ) );

    if ( empty( $categories ) ) {
        return $related_posts;
    }

    $limit = isset( $args['limit'] ) ? $args['limit'] : 4;

    // Query same-category products sorted by total sales
    $same_category = new WP_Query( array(
        'post_type'      => 'product',
        'posts_per_page' => $limit,
        'post__not_in'   => array( $product_id ),
        'post_status'    => 'publish',
        'meta_key'       => 'total_sales',
        'orderby'        => 'meta_value_num',
        'order'          => 'DESC',
        'tax_query'      => array(
            array(
                'taxonomy' => 'product_cat',
                'field'    => 'term_id',
                'terms'    => $categories,
            ),
        ),
    ) );

    $related_ids = wp_list_pluck( $same_category->posts, 'ID' );

    // If we need more, add products from parent categories
    if ( count( $related_ids ) < $limit ) {
        $parent_cats = array();
        foreach ( $categories as $cat_id ) {
            $parent = wp_get_term_taxonomy_parent_id( $cat_id, 'product_cat' );
            if ( $parent ) {
                $parent_cats[] = $parent;
            }
        }

        if ( ! empty( $parent_cats ) ) {
            $parent_related = new WP_Query( array(
                'post_type'      => 'product',
                'posts_per_page' => $limit - count( $related_ids ),
                'post__not_in'   => array_merge( array( $product_id ), $related_ids ),
                'post_status'    => 'publish',
                'meta_key'       => 'total_sales',
                'orderby'        => 'meta_value_num',
                'order'          => 'DESC',
                'tax_query'      => array(
                    array(
                        'taxonomy' => 'product_cat',
                        'field'    => 'term_id',
                        'terms'    => $parent_cats,
                    ),
                ),
            ) );

            $related_ids = array_merge(
                $related_ids,
                wp_list_pluck( $parent_related->posts, 'ID' )
            );
        }
    }

    return array_slice( $related_ids, 0, $limit );
}
add_filter( 'woocommerce_related_products', 'woosell_smart_related_products', 10, 3 );