Get values from node before checkout process

Hi,
I have a contest site where users post puzzles and set prizes for these puzzles.
I want the user to pay for publishing the puzzle and pays for the prizes he/she sets.
Using this wonderful module, I'm able to collect money for node publishing; however, I'm unable to add the prize money in the checkout.
My problem is that this module workflow is different from the normal commerce products; I tried many of the donation tutorials but all of them depend on the product poster is the site admin.
I tried adding a "prize value" field in the node to be published but I can't link this field to the line item of the product.

Any help would be greatly appreciated.
Many thanks in advance.