How are shipping costs calculated for multiple items?

Currently, the shipping methods available on Printliss include; the flat rate shipping, the weight based shipping, and we also offer expedited shipping which attracts a different price.

Flat rate shipping example:

If you order 3 shirts, you pay $3.00 x 1 + $1.50 x 2 = $6.00. So, you pay $3.00 for the first item and $1.50 each for the other two items.