Thanks! Well, it's up to the webshop to verify the amounts by calling the - well - callback URL. 
I'm working on server-side validation as well BUT the goal is to have a **decentralized** payment gateway.

In theory, this means the payment gateway should not rely on anything or anyone to be functional, so the end user needs to verify the amount payed against the amount he requested.

Amount verification will be included for services that run on our own node of course