EE4 Promotions simple Gift Certificate system. This snippet reduces the value set on a promotion code if is it as 'dollar discount' by the total amount of the transaction it is applied to, once the value reachs 0 it sets the Valid Until date of the promotion code to 'now' so it can no longer be used.

tw_ee_poc_promotions_gift_cert.php

<?php // Please do not include the opening PHP tag if you already have one.

// This snippet reduces the value set on a promotion code if is it as 'dollar discount' 
// by the total of the transaction it is applied to, once the value reachs 0 it sets the 
// Valid Until date of the promotion code to 'now' so it can no longer be used.
add_filter('FHEE__EED_Promotions__add_promotion_line_item__bypass_increment_promotion_scope_uses', 'tw_ee_poc_promotions_gift_cert', 10, 4);
function tw_ee_poc_promotions_gift_cert( $bypass, $parent_line_item, $promotion, $promotion_line_item) {
    // Pull the related EE_Price object.
    $promotion_price = $promotion->price();
    // Check we have a valid EE_PRice object and the promotion is NOT a percentage discount.
    if( $promotion_price instanceof EE_Price && !$promotion->is_percent() ) {
        // Pull the users checkout from SSN.
        $checkout = EE_Registry::instance()->SSN->checkout();
        // Check we have a valid EE_Checkout object.
        if ( $checkout instanceof EE_Checkout ) {
            // Pull the EE_Transaction from checkout.
            $transaction = $checkout->transaction;
            // Check we have a valid EE_Transaction object.
            if( $transaction instanceof EE_Transaction ){
                // Reduce the promotion value by the current transaction total.
                $new_promo_price = $promotion_price->amount() - $transaction->total();
                // If the sum of the above is negiatve, set the promotion value to 0.
                $new_promo_price = max($new_promo_price, 0);
                // Set the amount on the promotion price object.
                $promotion_price->set_amount($new_promo_price);
                // Save the changes to the DB.
                $promotion_price->save();
                
                // If the promotion value is now 0, set the value until date to now and save.
                if( $new_promo_price === 0) {
                    $promotion->set_end(date('Y-m-d g:i a'));
                    $promotion->save();
                }
            }
        }
    }
    // Setting $bypass to true will bypass the 'uses' value increment when a promotion code is used, 
    // leaving it false will not bypass the increment.
    return $bypass;
}