How do I change shipping rates for the same item to use a different box size?

[MWPhotography]

I am a photographer and sell prints online. I have several prints you can buy. I sell from 4x6 inch to 16x20 inch, everything 11x14 and under ship in a flat mailer, but the 12x18 and the 16x20 ship rolled in a long narrow box or tube. 

How do I set 2 different shipping prices based on the same item in my inventory?

[Goldneye]

HI, you can charge a Flat Rate shipping fee for all your items and add the additional charge to the price.

Example of similar charge, when the customer selects the 4XL size the additional fee is added.

Square Support page 

https://squareup.com/help/us/en/article/6910-set-up-shipping-areas-and-rates-with-square-online-stor...