We have been using osCommerce since 2004. I would highly recommend this cart. There are a huge amount of modules available to suit your requirements. I can’t speak to US Mail but Canada Post fully supports OSC on their servers, so I assume US Mail would be the same. There are several different payment options available, I choose to bypass this with the cheque / money order method. I don’t have any desire to waste time worrying about credit card issues. In five years I have had one issue with a spammer screwing with the reviews module, I simply disabled Reviews and had to delete any review pages. No problems since. Occasionally you get harassed by some entrepreneur trying to sell their OSC customization services, this doesn’t happen all that often.
Customization can be a bit tedious unless you are very comfortable in PHP but then again their community forum support is first rate. When I began the process I knew next to nothing about PHP and struggled to figure out the interdependence of all of the pages. It can be a real pain in the butt trying to figure out which page generates a specific line of text.
Once everything is set up and all your products are entered there is little else to do except process the orders as they come in. In the last 2 years I would say I have spent under 20 hours in modifications.