WooCommerce – Only allow certain product combinations in cart

woocommerce-restrict-cart-product-combinations.php

// Step 1 - Keep track of cart contents
add_action('wp_loaded', function() {
  // This variable must be global, we will need it later. If this code were
  // packaged as a plugin, a property could be used instead
  global $allowed_cart_items;
  // We decided that products with ID 737 and 832 can go together. If any of them
  // is in the cart, all other products cannot be added to it
  $restricted_cart_items = array(
    737,
    832,
  );

  // Use a separate cart instance, so that the one used by WooCommerce is unaffected
  $cart = new WC_Cart();
  foreach($cart->get_cart() as $item) {
    if(in_array($item['data']->id, $restricted_cart_items)) {
      $allowed_cart_items[] = $item['data']->id;
    }
  }

  add_filter('woocommerce_is_purchasable', function($is_purchasable, $product) {
    global $allowed_cart_items;

    // If any of the restricted products is in the cart, any other must be made
    // "not purchasable"
    if(!empty($allowed_cart_items)) {
      $is_purchasable = in_array($product->id, $allowed_cart_items);
    }
    return $is_purchasable;
  }, 10, 2);
}, 10);
// Step 3 - Explain customers why they can't add some products to the cart
add_filter('woocommerce_get_price_html', function($price_html, $product) {
  if(!$product->is_purchasable()) {
    $price_html .= '<p>' . __('This product cannot be purchased together with "Product X" or "Product Y". If you wish to buy this product, please remove the other products from the cart.', 'woocommerce') . '</p>';
  }
  return $price_html;
}, 10, 2);

Absolutely, it's gotten me on my way and I'm learning so it's now up to me to implement what I need :) thanks again!