What is the best way to connect automatic payment to a form?

Question

Payment systems

Hello. Actually, the task is this.
There is a regular site with a form where the client can fill in contact information, and after that, in theory, he should be redirected to the payment system, where he can pay for the service.
In fact, everything is simple. One service, one price. How is it safer, faster, and without problems to connect online payment to a simple html form? Robokassa, Interkassa, what do you recommend?