How can I integrate a lightweight payment process in my wordpress site? ¿WPe-Commerce?

I am on a wordpress website that is not a shop, the website offers information in maps. The issue is that the website offers a service of creating legal contracts. The logic is that a registered user gives the personal data, contract data, etc and then the owners of the website make a legal contract.

The point is that this action must be paid by the customers and I want to implement payment process.

I saw that I good option is to use Gravity Form with its payment plugins, but the prices is high.

I saw also that there is another way with WPe-Commerce, but I don't know if it's convenient to use for this purpose.