There are 2 options you can try, both are good but it depends on your preference.
Option 1 "Add Multiple Products to Cart at Once" Link: http://www.oscommerce.com/community/contributions,738
Option 2 "Add Multiple Products" Link http://www.oscommerce.com/community/contributions,1129
The instructions are pretty easy to follow, just be sure to back up your files first since it's easier to restore rather than go back and try to undue any mistakes.
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