The postage has been set to be the same on all shirts, so I'm not sure which shirts you are seeing different pricing on, but please PM me so I can fix it. The postage is different for the shipping destination for a given shirt.
The postage accumulates because it is based on weight when I ship via First Class Mail package up to 13 ounces (~2 shirts) at 13 ounces, there is a jump to regular package rates. The initial shipping charge to the US is $4.00, with an additional $2.50 for each additional shirt, therefore, shipping is $6.50 for 2, $9.00 for 3, $11.50 for 4, and so on.
This is dead on what my shipping costs are.
Now making T-shirts! Want custom designed/sized die-cut stickers or shirts?? email me firstname.lastname@example.org