I have an existing site that is using standalone PHP pages to do the following.
Step 1. User enters VIN Number
Step 2. "Teaser" VIN number results (if found) are returned along with "Buy Now" button.
Step 3. Clicking "Buy Now" directs user to Paypal Payment page.
Step 4. Upon receipt of payment, user is routed back to site, another API call is made to 3rd party site, and VIN Number results are returned as downloadable PDF file.
The existing process working, but now I wish to handle the payment processing on my account using WPP (Paypal and SSL are already installed). This means a session is required to keep the VIN number, and upon transaction "success", another API call is made to 3rd party site and PDF download is provided. All API calls and PHP files are available, so there shouldn't be too much work trying to figure out correct procedure for 3rd party API call.
I'm ok to the idea of using plugins or themes such as https://easydigitaldownloads.com/ that make this process easier.
Feel free to PM me for details on existing site flow.