Can also use a negative price! I wonder what would happen if you went ahead with that. Presumably the payment gateway wouldn't pay me $5. Seems a strange choice to be having those parameters in a GET.
Having dealt with payment gateways, I would not assume that.
I can "change the price" of anything on Amazon with dev tools, but that won't help me when I go to buy it. I appreciate the concern because amazingly this has been a real vulnerability on sites before (recently on a crypto exchange I think).