Woocommerce cart checkout only if specific quantities - php

Question

In Woocommerce, I would like my customers to buy either 18 or 36 products but not between the two. I wanna remove the possibility to checkout from 19 to 35.

Answer 1

You told me in comment that you don't know php well. Most of the time we don't offer php code here, but I found it interesting to do. And it may helps some people in the future.
As I said, one possibility is to do a redirection to the cart page on the checkout page if the customer doesn't have 18 or 36 products. After the redirection on the cart page, you must display a message for the customer to see that he must have 18 or 36 products.
Summary
add_action on template_redirect to check the product count and handle the redirection. If we it redirects, add an arg like invalid-cart-product-count in the url with value equal to the product count.
add_action on woocommerce_before_cart to check if we get the invalid-cart-product-count so we display a message.
Working code (tested)
There is the code that you need to place in your functions.php file of you theme or child theme (preferred option).
/**
* Check on template redirect the cart product count
*/
add_action('template_redirect','product_count_template_redirect');
function product_count_template_redirect() {
// If we are on checkout and the cart contents count is not equal to 18 or 36
if(is_checkout() && !in_array(WC()->cart->get_cart_contents_count(), array(18, 36))) {
// Redirect on the cart page and add a query arg to the url so we can check for it and add a message
wp_redirect(esc_url(add_query_arg('invalid-cart-product-count', WC()->cart->get_cart_contents_count(), wc_get_cart_url())));
exit;
}
}
/**
* Handle the message on the cart page if we have the 'invalid-cart-product-count' arg in url
*/
add_action('woocommerce_before_cart', 'cart_page_message');
function cart_page_message() {
// If its set and not empty
if(!empty($_GET['invalid-cart-product-count'])) {
// Display the message with the current cart count and the count that user need to have
$message = "<div class='woocommerce-error'>";
$message .= sprintf(
__('You currently have <strong>%s</strong> products in your cart. You must have <strong>18</strong> or <strong>36</strong> products in your cart to be able to checkout', 'your-text-domain'),
// Force the arg to be an int, if someone malicious change it to anything else
(int) $_GET['invalid-cart-product-count']
);
$message .= "</div>";
echo $message;
}
}
Tell me if you don't understand or if this not exactly what you need.