Hi Ta,
If you mean selling physical items that have nothing to do with what PrintShop Mail Web can generate, I would steer clear of this. PrintShop Mail Web is not sold as a complete shopping cart solution, this is not support nor recommended.
I would suggest looking at the
PrintShop Mail Web Integration Guide, to learn how to integrate PSMW into an external system instead. This would let a user make their order in PSMW and then be redirected to the external system, where you could automatically offer similar products, accessories, free items, etc.