I have experience with services such as Zazzle. The problem with them is that during the purchasing process your customer, effectively, discovers that they can have that t-shirt or coffee mug printed for less if they just setup an account with them.
Hypothetical case: You manufacture roller skates and sell them online. You want to offer your customers the ability to order --from you-- t-shirts, coffee mugs and other merchandise that is customized in some form. While this is not your main business it could represent a non-trivial revenue stream.
The problem is that if you go to a service like Zazzle (not picking on them but it is a really good example) you, effectively, expose your customer to the ability to bypass you entirely and order their own custom merchandise through that service.
The ideal t-shirt and merchandise provider would be one that disappears in the background and simply takes and fulfills your orders.
How does one do that?