Snippet to automatically add products to WooCommerce cart

add-products-to-cart.php

<?php

// FUNCTION TO CHECK IF ITEM IS IN CART
function item_in_cart( $product_id ) {
	$found = false;
	if ( sizeof( WC()->cart->get_cart() ) > 0 ) {
		foreach ( WC()->cart->get_cart() as $cart_item_key => $value ) {
			$product = $value['data'];
			if ( $product->get_id() == $product_id ) {
				$found = true;
				break;
			}	
		}
	}
	
	return $found;
}	

// ADD PRODUCT TO CART BY DEFAULT
add_action( 'init', 'aaptc_by_default' );
function aaptc_by_default() {
	if ( ! is_admin() ) {
		$product_id = 12986;  // Replace with id of product you want to add
		if ( ! item_in_cart( $product_id ) )
			WC()->cart->add_to_cart( $product_id );
	}    
}

// ADD PRODUCT B TO CART IF PRODUCT A IS IN CART
add_action( 'woocommerce_add_to_cart', 'aaptc_based_on_product', 10, 2 );
function aaptc_based_on_product( $item_key, $product_id ) {
	if ( ! is_admin() ) {
		$productA = 34; // Replace with id of product a
		$productB = 56; // Replace with id of product b

		if ( item_in_cart( $productA ) ) {
			if ( ! item_in_cart( $productB ) )
				WC()->cart->add_to_cart( $productB );
		}	       
	}    
}

// ADD PRODUCT TO CART IF CART TOTAL REACHES 500
add_action( 'template_redirect', 'aaptc_based_on_total' );
function aaptc_based_on_total() {
	$cart_total	= 500;	

	if ( WC()->cart->cart_contents_total >= $cart_total ) {
		if ( ! is_admin() ) {
			$product_id = 12986;  // Replace with id of product you want to add
			if ( ! item_in_cart( $product_id ) )
				WC()->cart->add_to_cart( $product_id );       
		}
	}        
}

?>