well, if you are good with coding and such, you SHOULD consider OSCOMMERCE then.> They have a LOT more options as far as contributions.. For things like Screen Printing, you might want Special Pricing Per Customer options, so different customer groups and see ONLY their pricing.. So when someone sets up an account, you give them a customer group which shows them THEIR price.
Also Quantity Price breaks, so if you sell one shirt for $7.99 you can sell 10 for $45.00 and so on.. You can set the quantity and the price.. There is also a MULTI STORE contribution where you can have separate stores for the different types of printing.. SO one store for shirts, one for signs, one for paper BUT once the customer signs up on one store, they become a customer of all of them..
There are LOADS of things you can do.. And although i didnt do TOO much with zen cart yet, i know that it is an offshoot of oscommerce, but since its newer, there are not as many contributions..
it basically depends on how far you want to get into it