When I was using the MachForm Version 2 (ages ago), I was able to include the payment link in my confirmation email. If the user wasn't able to complete payment, they could use the link to pay after the fact, or I could send it to them. Later versions, took that capability away. I was also able back then to add code to the post-functions.php file to check if they'd already registered and return an error. That also went away.
I've asked several times for a way to do this with the new code base. I haven't received any answer. :-1_tone2:
If anyone has a solution, I'd love to hear it!