Same (selling merch of my art through Printful).
I went through the hassle of creating my catalogue of items on Printful, connecting to Squarespace but haven't been able to come up with a shipping rate that doesn't either rip off my customers or screw myself.
I think I'm going to downgrade my Squarespace account (not pay for the eCommerce tier) and just link out to my RedBubble. They handle billing much better anyway.
#givingup