Many people seem to have a problem with customers that pay with PayPal not returning to their website. When their customers do this, the store owner receives payment, but does not receive the order information.
A simple solution to this -- which some people may not like, but it should work fine -- is to simply put a message at the payment selection page advising the customer to make sure they return from the PayPal site to this site to complete their order.
This should eliminate MOST of the PayPal order problems that result from the customer not returning to the site. I have seen this implemented on a site that was for a pretty big, multi-national company. I didn't think it was unprofessional at all. That was even before I knew about this problem, or osCommerce, for that matter.
I cannot say how effective this will be, because our webstore is still not operational. It is still in the testing stages. We hope to make it live later today or tomorrow if all goes well. So, since we don't have actual experience with this, I can't say how well this will work in reality. It works fine in testing.
WE ARE USING THE STANDARD PAYPAL MODULE THAT COMES WITH OSCOMMERCE 2.2MS2.