You have to install a french credit card system (Cyberplus, with Atos) on oscommerce creloaded.
Orders must be recorded in OScommerce even customer doesn't go back to the shop.
You have to fix template problem : When a 404 error occurs, sessions don't work.
Cyberplus must work fine even template get a 404 error with pictures (because designer could do some mistake).
If this is a server's problem you have to give your instructions to sysadmin to fix it.