External form submission data lost

Hello everyone, I am developing a plug in specifically for a client of mine. To make a long story short, here’s my setup and issue.

I have a shopping cart that when they click checkout, they are taken to a third party to process payments. Once payment is processed, the customer is redirected to a URL of my choosing with some POST variables that contain information about the transaction.

Here’s my problem, those POST variables are not making it back to my shortcode on the client’s website. However when I have the third party redirect the customer to a non-Wordpress website, I have no problem accessing the POST variables. I’m betting WordPress has some builtin form submission redirect to prevent multiple submissions on refreshes? If I’m right, is there anyway to disable that?

Thank you in advanced!!