If you have a promotion that you're offering, such as “purchase x product and receive x off of x product”, you'll want to make sure that the user can't override the purchase requirement by adding the required product, receiving the discount on the second product, then removing the original required product.

This snippet looks for the presence of a specific coupon (based on the code), and the total quantity of any products with a specific code. Then if the coupon is there but the products are not, it removes the coupon. If it has the products, but not the coupon, it adds it. This logic is run on page load of the checkout, along with whenever a product is removed, quantity updated or coupon added on the checkout. This should then ensure that the coupon is only present when the required conditions are met.

The script is based on the quantity of a specific product code - but it can be adjusted as needed, whether you need it for a specific category of products, or a specific total order amount of specific products rather than quantity.

You can apply the snippet below within the “configuration” section of your store's FoxyCart administration - look for the “Add custom header and footer code to your templates” option, enable it (if not already) and include the javascript snippet in the footer.

{% if context == "checkout" %}
<script>
FC.client.on("ready.done", checkPromoProducts);
FC.client.on("cart-item-remove.done", checkPromoProducts);
FC.client.on("cart-item-quantity-update.done", checkPromoProducts);
FC.client.on("cart-coupon-add.done", checkPromoProducts);
 
function checkPromoProducts() {
    var coupon_code = "COUPON_CODE",
        product_a_qty = 0,
        coupon_id = 0;
 
    if (FC.json.coupons.hasOwnProperty(coupon_code)) {
        coupon_id = FC.json.coupons[coupon_code].id;
    } 
 
    for (var i = 0; i < FC.json.items.length; i++) {
        if (FC.json.items[i].code == "PRODUCT_CODE") {
            product_a_qty += FC.json.items[i].quantity;
        }
    }
 
    if (coupon_id > 0 && product_a_qty == 0) {
        // The coupon shouldn't apply anymore
        FC.client.request("https://" + FC.settings.storedomain + "/cart?cart=remove_coupon&coupon_code_id=" + coupon_id).done(function(dataJSON) {
            FC.cart.render();
        });
    } else if (coupon_id == 0 && product_a_qty > 0) {
        // They should have the coupon, let's add it
        FC.client.request("https://" + FC.settings.storedomain + "/cart?coupon=" + coupon_code).done(function(dataJSON) {
            FC.cart.render();
        });
    }
}
</script>
{% endif %}

You'll also need to update the coupon code string in the script (it's near the top as “COUPON_CODE”) and the product code string too (shown as “PRODUCT_CODE”) to the coupon code you created.