Buying $50 to fix bug in a osCommerce script

I think it's really easy for someone with PHP knowledge. Here is a copy-paste of what I posted in the osCommerce forums:

So this is what happens when someone checkout:

1. You go to the "Delivery Information" page (checkout_shipping.php), chose your shipping and click continue.
2. You go to the "Payment Information" page (checkout_payment.php), you fill out your coupon code and chose your Payment Method.
2a. If you chose PayPal Express you get redirected to PayPal.com and the coupon code you typed in will not be processed because you are sent away from the webshop.
2b. If you chose Credit Card as your method of payment it takes you to the "Confirmation" page (checkout_confirmation.php), where your discount is applied and you have a new total.

2ba. When you click "Confirm" you pay the new total, and everything works perfect.

So in some way we need to have it set the new total BEFORE sending the customer on to PayPal.com... I don't think it is that hard to do, but my knowledge is limited.

 

PM sent looking forward to hear from you.

 

I am sending you a PM right now.

 

Feral gave it a go, but couldn't solve it for me. Anyone knows what they're doing? I don't want to give away any access or passwords as I have been ripped off before...