zshopes,
I just went though the same process. I was able to read a lot of previous posts, and I got it working.
This is not a step by step tutorial, just a couple keys things that held me up.
Like many others have said, you do not need to enter the key or certificates near the bottom. Just leave them blank.
The big thing that held me up was when I switched to Sandbox to test it out I left my real email address (not my custom Sandbox given address) in the email field. Once I switched that it worked fine.
Do not over complicate things. It is much easier then it seems.
Here is a list of my settings.
Version: 1.0 (online status)
Visit PayPal Website (info)
Using the above link to signup at PayPal grants osCommerce a small financial bonus for referring a customer.
Enable PayPal Website Payments Standard
True
E-Mail Address
******_12875851_biz@**********.com
Payment Zone
--none--
Set Preparing Order Status
Preparing [PayPal Standard]
Set PayPal Acknowledged Order Status
Processing
Gateway Server
Sandbox
Transaction Method
Sale
Page Style
Debug E-Mail Address
info@*********.com
Sort order of display.
0
Enable Encrypted Web Payments
False
Your Private Key
Your Public Certificate
PayPals Public Certificate
Your PayPal Public Certificate ID
Working Directory
OpenSSL Location
/usr/bin/openssl