Besides doing mastering, on location recording, video production and restoration work my company also does limited run CD and DVD duplication for our clients when needed. That means quantities up to 100. Over that and we have a local duplicator that does very nice work that we can use. With our on CD printer, our two duplication towers and a color graphic's printer, capable of handling heavy paper, we have an investment of about $5K and we have more than paid off the equipment. With the demise of the CD in the offing this may not work as well now as when we got started with it 10 years ago.
Sure some mastering studios have connections with services that are related. I would be aware of muddying the waters though and taking on manufacturing/duping, printing T shirts sounds like one step to far removed to my mind. You know some duplication plants do mastering but I have heard of some getting less than stellar results.
Just be a little thoughtful.... are you a mix studio that offers mastering? a mastering studio that offers mixing? a duplication plant that offers mastering? etc. Side lining services could be treacherous. It is not a good look to be jack of all trades and master of none, if you can pardon the pun. I cannot see how a "do it all" place has the same day in day out focus and experience in mastering music unless you employ skill set specific people which is a commendable thing to do.
I am fortunate as my income is solely derived from mastering and my plan is to become known for doing one thing very well, so far it's worked and is continuing to work. Different strokes for different folks so the saying goes.