Frankly, your question(s) are always hard to follow, and have been answered hundreds of times before in these forums
>> Try performing a "Forum Search" using key words to pull up the many different answers that may provide a solution in a manner more understandable to you instead of creating duplicate threads!
Personally, I think osCommerce (although it has the functions you are looking for) is much bigger a beast to tame, and may prove too big for you to attempt using at this point without some professional help (based on all of your other questions and expressed understanding). It would mean starting over again completely.
If you want some real professional eCommerce assistance, email the OmniSource Team (affordable and precise). The simplest option is to "upgrade" to PayPal Pro....