Postage Price Calculator
If you look at the USPS Priority Flat Rate options:
$8 is good for 1-2 shirts but more than that you run into your package being over 1 pd so you would
need to move up the box to medium (est$11) to large (est$15.45).
I would go to the post office and get 1 of each flat rate and priority packages. This way you can see how much can fit into each box or envelope. Also, make sure you know which ones are flat rate and which ones are priority.
If you look at the USPS Priority Flat Rate options:
$8 is good for 1-2 shirts but more than that you run into your package being over 1 pd so you would
need to move up the box to medium (est$11) to large (est$15.45).
I would go to the post office and get 1 of each flat rate and priority packages. This way you can see how much can fit into each box or envelope. Also, make sure you know which ones are flat rate and which ones are priority.