I'd like to be able to create an order for say $5,000 and then charge monthly based on the results I drive for the client until the $5,000 is reached.

Based on our agreement, the amounts could differ each month. It could be $500. It could be $200 or $1,000. But the payments would measure against the $5,000 maximum until they hit the agreed upon result.

Essentially, take the existing payment link, and input a custom amount there towards the overall invoice.

I could see this being useful for base + performance as well.