A business logic vulnerability exists in the Add to Cart function of itsourcecode Agri-Trading Online Shopping System 1.0, which allows remote attackers to manipulate the quant parameter when adding a product to the cart. By setting the quantity value to -0, an attacker can exploit a flaw in the application's total price calculation logic. This vulnerability causes the total price to be reduced to zero, allowing the attacker to add items to the cart and proceed to checkout.