Pay "open" service fee/invoice amount

We need a way for customers to pay invoiced amounts on a SquareSpace site. Essentially a service with a "filled in" dollar value. We were originally told we could use the Donation Block for this but the "Donation" and "Contribution" labels cannot be modified. 

Process we need:

1. customer receives invoice
2. goes to website
3. enters form information
4. enters payment amount
5. payment processed
6. customer received email receipt
7. client receives email notice.

The donation block actually has the function we need we just can't  modify the current labels in SS.