Submit form to 3rd party site, receive response to update original form What I want to do

We have a simple form that people fill out to attach documents for review.  We want to monetize it, using a specific vendor we currently use for non Drupal sites.  

What we want to add is when someone saves their form, go to a confirm page, stating the price, then submit this to the 3rd party site.  They process the payment and post a response back to our site, which we then modify the node to update the status to paid.  We currently do this using Google app scripts on many other sites.  We want to keep this site in Drupal for other reasons.

I'm very new to drupal, and am having some trouble with how to submit the form, and especially how to receive the post from the 3rd party vendor.  I feel this isn't a crazy way to do this, but the lack of modules that 

We've looked at Drupal Commerce and Ubercart, etc, but they seem pretty large for our needs.
There are a variety of ways you can do this using the 3rd party API and the hooks provided by Webform such as 

http://www.drupalcontrib.org/api/drupal/contributions%21webform%21webform.api.php/function/hook_webform_submission_presave/7