How to securely connect payment systems to the site?

Question

PHP

Hello everyone.
I am trying to connect a form of boards. systems to the site.
Payment service. The system issues a form with input type hidden fields and is immediately submitted to the system website (via post).
Nothing prevents the user (having knowledge of html) from changing the payment amount and submitting the form on the payment site. system to pay the sent(changed) amount.
How to protect yourself from such situations?
Thanks in advance for help.

Answer 1

When you receive a response from the payment system about the status of payment - immediately check the amount paid. If it does not converge - write to the user about it.