That is by design, but it only affects our Buy Now buttons, which are intended to bypass our cart and take buyers directly to instant checkout for just one item at a time.
If you are using our standard Cart buttons, an &amount= parameter appended to those button URLs would only ADD that amount to the item's base price set in Seller Admin.
Either way, when our system receives notification of completed payment, it checks the price paid against the item's price defined in Seller Admin, and if the payment undercuts the set price, we WILL NOT process the sale and instead send you a Suspected Fraud notification email, so you can investigate and decide how to handle the situation yourself.
