Surely the pins are cheap but your running shirts will be damaged by pinholes? And what about a running jacket with holes?
Sounds almost like our product. We can do the same.Our product is a plastic shell with a small magnet inside.
Companies can buy our product with company logo or running clubs with club name, etc.
http://www.sportsmagneter.dk/english-version
Magnets instead of safety-pins...
Check my website. I have a solution to your problem with safety pins.
Only 16 grams total weight and no rubbing on the inside of the t-shirt.