So you would just like to add commerce to this form page? So long as you can edit the source php code on the page directly you can use eCart to add in an add to cart server behavior and button that will get the item or items into the cart. Once you do that you can then create a cart display page and generate a checkout with the checkout wizard and choose Payments Pro as the checkout option.
You should checkout the support page for eCart and have a look at the getting started guide.