If you're paying an average of $8 to ship a shirt, you're losing money this way. It costs me roughly $3 per shirt, give or take a little depending on the final weight. Even if 1 in 100 shirts were to arrive damaged (which hasn't been the case) and you had to refund, you still come out ahead. From a customer service perspective, you may still prefer your method, but from a purely financial perspective, envelopes are the way to go. If you're really worried about the bag getting torn, use a tyvek bag instead of the cheap poly ones. Tyvek is pretty tough to rip and still far less than a box.