Fix for Authorize.net posting to ecommerce site

We have an ecommerce site. When someone makes a purchase, they leave the site and go to [login to view URL] to fill in their credit card information. Then, [login to view URL] is supposed to post back to the site with a success message. The problem is that the transaction is being approved through [login to view URL], but then the user gets an error because Authorize apparently cannot "post back" to the web site. Our host, Rackspace, is showing NO errors. They say it's not on their end. [login to view URL] says it's not their problem, that it's a problem in the code of the site somewhere. Here is what [login to view URL] says, and what I need you to do to fix the problem: e transaction is complete. The problem is in the posting back to the site. Once you put your cc info in, [login to view URL] approves the transaction, then gives an error that it can't post back to the site." [Caroline] "[login to view URL] says it's a script permission or something on our site causing the problem." 1) Check that your script permissions are correct and that it can accept an HTTPS POST. 2) Check that the script is not completing other functions before writing to the screen such as writing to a database or sending e-mails. 3) Please check to see if there are different processes that are used in your script for approvals, declines, or errors. Check each process to be sure that they write to the screen before any other functions. Please don't bid unless you are familiar with this and truly feel you can fix it. I have sent it to several coders already who end up not understanding or being able to fix it. I also need this done ASAP. Today if possible! Thank you.