I offer banners but I don't print them. I work very closely with a banner/sign shop right up the street. They get shirts orders that I discount to them and I get banner orders that they discount to me. It works very nicely. You just need time to build a relationship like that.
I am a firm believer in offering anything you can to your customers. You don't want them shopping elsewhere. Because if they call me for balloons I am going to try and get their shirt, sign, banner, pen, water bottle....... orders too. I also believe that if you are great at printing shirts, print shirts and let me do the balloons. When I need pens I call a pen company. When I need golf balls I call a golf ball company...
Just my opinion.
Matt