Add-on to credit payment to gravity forms total?

Any help and I'll be eternally grateful!

I'm trying to solve an issue for my website/small business.
The way I have it set up right now, I have an "order form" that requires payment to submit (it also requires that the user be logged in so that each submission can be linked to a user account)

I am looking to run an ad, so to improve potential conversion, I am wanting to shorten the steps to payment for first time customers. Is there a way (possibly another plugin) that works with gravity forms and would allow users to pay in one step, then I could prompt them to register an account and it would credit their prior payment toward the order form once they get to that step?

I was thinking about trying to find a coupon generating addon (I think gravitywiz might have one?) But that feels cheap, so if I could create an account credit, that would be really neat.

I suppose the issue then becomes, that they aren't logged in, so are we able to assign the credit based on an email or other metric that they will register with anyway?

Any ideas? Am I making making sense?