Coupons, Partial Payments (deposits) and PayPal Checkout

I am trying to figure out the best way to do this, but so far every solution has a drawback.

We are selling tours and require 30% deposit to be paid upon form submission with PayPal Checkout.

Customer can use coupons, and the deposit and remaining balance must be calculated against the net price after discount.

I can calculate the deposit in a number field, but that number field cannot be used for a Payment in PayPal Checkout (just product and total fields).

If I use a product field for the deposit, it gets used in the calculation for the net price and it screws up my calculations.

If we did not have the coupons, I would create a product for the balance that would be -.7 * the net price. Then the form total would be my payment, but using the coupon makes that not work for the reasons I explained. I ended creating two extra products, deposit which is a calculated product with the deposit value that I set as the payment field, and then another product with the negative deposit value to counteract and fix the total. That works, but leaves me with fields to hide on the form and in the order summary.

So basically I need this:

Product Price: $1,000
Coupon: Test10 (10%): $100
Net Price: $900
Deposit (30%): $270 ← This will be the payment when I submit the form
Remaining Balance (70%): $630

Is there a better way to handle partial payments when a coupon can be involved? Or am I just using the wrong tool for this?

There was an old post on this topic, but it did not have coupons.

Thanks!