Here is a test page for the PayPal payment.
Here’s a test payment dropdown for the “Shopping Cart” version; we would have to actually enter payment amounts (which might be different in every case?), so it seems impossible, unless you want to give people flat amounts to pay and credit them:
Here is a single-item payment option, with no dropdown. People can enter the amount at the Paypal site themselves – but the quantity is always “1.” There is no way to choose multiple amounts – so people could pay their monthly bill, but would have to do the math themselves.
I’ve found another practice that does this. They say “Please use this first button to submit payment for counseling by clicking “Pay Now” and then entering the amount you wish to pay toward past, current, or future appointments.”
Would that work for you?