Osc Install 1 Page Checkout
Install One Page Checkout
http://www.oscommerce.com/community/contributions,6646/category,all/search,one+page+checkout
and Vouchers / Coupons Module
http://www.oscommerce.com/community/contributions,282/category,all/search,vouchers
And make it work on an oscommerce store which already has many contributions installed including MVS (Multi-Vendor-System) and Country-State Selector.
Requires a real oscommerce expert to get this to run with the currently installed contributions,