Just a simple checkout page

I’m new to wordpress and I’m trying to help a friend. She has downloaded the wp-commerce and gold cart plugins to her wordpress site.

She is not selling products but wants to allow her clients to enter an amount (any amount) in a balance input box and pay via credit card. She has an account with authorize.net. I created a button from the simple checkout option by authorize.net and embedded it in one of the pages of her site. It works and goes straight to authorize.net. but I rather create a form (which I did) that looks like a part of her site. I have a “pay balance” button at the end of the form.

In the interested of saving time and not driving myself crazy…Can I just trigger a payment function from the gold cart plugin using the submit event that I created in the form? Or is there a short code that I can embed that goes straight to the checkout (since there are no products) and i can code in a balance box?

Thanks.