Well my friend I am in the same boat.
I will never buy a product form this people again. They sell product and do not offer support. I feel like I have been scammed.
I just purchased express account 2 weeks ago, I thought I would have had a few months of support but when I click on send support request, they ask for a payment of 33 dollars.
Now I am left like you with something that does not work, over 350 people looked at your question, noone replied. Not even one of the support guy took 2 minutes of his time to reply to your question.
I went on their help page and it says(for express invoice which should be similar):
You should already have a PayPal account.
Enter the API Username and Password provided to you from PayPal.
The Gateway URL is https://api-3t.paypal.com/nvp, though you may want to test the gateway in the sandbox using the URL https://api-3t.sandbox.paypal.com/nvp.
For API Credentials, you will select 3-token Authentication and enter the signature provided by PayPal. If you have your own SSL certificate, select Client Side SSL, enter the certificate password and browse for the certificate file.
I tried both, nothing works I keep receiving the message "the credit card gateway is not properly configured.
Anyone with a bit of knowledge please?
Thank you,
Ben