Shipping is all dependent on weight, size, and destination. If you are sending out one shirt, then it can go out USPS First Class Parcel for around $2.50-300, two shirts via a USPS Priority Flat Rate envelope is $4.80. You can do probably up to 4-5 shirts or so in a flat rate box for $9.80. If you want tracking numbers or insurance, the post office or mailing place will add additional amounts for those.
Larger packages that do not fit into flat rate boxes are generally cheaper through a carrier such as DHL, FedEX, or UPS and include a 100 insurance and tracking automatically.
Each of the services have an online rate calculator that will get you an idea of what to charge.
USPS - The United States Postal Service (U.S. Postal Service) Shipping, Logistics Management and Supply Chain Management from FedEx UPS Global Home www.dhl-usa.com