I have made shirts for a couple of years for clubs and groups and probably do it in an unusual way, but they work. I'm an aficionado, but I know the process. This is how many printers can organize your screen printing designs anyway, and I like the greatest possible power. I work primarily in groups with their own printers and sketchy consistency. To make it as simple as possible, I do what I can.
Offer them just a copy of a completely sized 300dpi file, possibly a.psd, if you do a raster image. This can be done by most printers. This form of printing is normally however much more costly and depending on the printer will come in a lower quality.