I solved the "Error: Invalid Merchant or Merchant doesn't exist!" problem in the weirdest way you can imagine. All I did was move the info box from the right side of the page back to its default place on the left side!
In the middle of March I installed two OSCommerce 2.3.3 stores and got them both working with PayPal Payflow -- I have the new "Advanced" account. I even put through several small transactions on my own credit card to make sure everything was working.
After that, I continued to make changes in the store appearance, and in the process PayPal Payflow stopped working in one of them.
I spent at least 12 hours trying to solve the problem. I restored the files that I had changed. I spent about 3 hours on the phone with the PayPal tech staff, convinced that the problem must be on their end. I even tried installing a new user on the manager.paypal side. They were convinced that the problem must be on my end, and I only half believed them.
Since one of my stores was working and the other wasn't, I decided to make everything
in the non-working store identical to the one that was working. So I downloaded all of the oscommerce-2.3.3 files (except the configuration files) from the store that was working and uploaded them to the store that wasn't working. But it still didn't work.
So I kept looking for any difference, any difference whatsoever, between the two stores. And the only difference that I could find was that I had moved the info box in the non-working store from its default place on the left side of the page to the right side of the page. So I moved the info box back to the left side and, voila, it worked!
I am about 90% sure that this was the solution, but I am not positive, and I am not
going to check by putting the info box back on the right! Now that it's working, I'm not planning to change anything
Edited by HowardR, 01 April 2013 - 08:51 PM.