Looking for a simple solution for getting payment from a customer

I have a client who is looking for a way for a customer to go to their site and pay with a credit card for their invoices. They just want to them to fill out a basic form (name, phone, invoice#…:wink: and enter their credit card info and the amount they wish to pay.

I tried using gravity forms but their merchant processor (gateway) only supports Authorize.net through a Emulator. I learned that gravity forms does not support this function. I did go into the php and try to edit the code to submit to the emulator but did not have any luck (most of the files say they are inactive that had the details the processor provided to change).

So my question is do you have any suggestions or ideas?