AS I understand it, Stewart & Heaton clothing are the only company who can do the Star on the shirts.....but they don't do any other embroidery.
So if you want something written on the shirt, you have to take it elsewhere to get that done.
Generally, Stewart & Heaton (one of the official CFS uniform suppliers) has a number of T-shirts in stock, so when you order them, you don't have to wait for them to be embroidered...
Pip
PS I don't work for Stewart & Heaton - I just had to buy some CFS T-shirts recently!