The WordPress Simple Shopping Cart plugin for WordPress is vulnerable to Insecure Direct Object Reference in all versions up to, and including, 5.1.3 via the 'process_payment_data' due to missing validation on a user controlled key. This makes it possible for unauthenticated attackers to change the quantity of a product to a negative number, which subtracts the product cost from the total order cost. The attack will only work with Manual Checkout mode, as PayPal and Stripe will not process payments for a negative quantity.
https://www.tipsandtricks-hq.com/ecommerce/wp-shopping-cart
https://www.tipsandtricks-hq.com/ecommerce/simple-shopping-cart-enabling-manual-offline-checkout