What’s the best third party site to sell photographs on Squarespace

Site URL: https://www.forallhumankind.com/stillsgallery

Am looking to sell some of my still images. Is there a good third party site that prints and ships and works seamlessly with square space? 

Here is one:

https://www.gelato.com/en-US/api/print-on-demand/squarespace/?gclid=CjwKCAjwzt6LBhBeEiwAbPGOgcQFFiLGugzKe63H1-2BHvTRQoCPSWRkhleJBCW94x1vxqV5xOdKjxoCmpoQAvD_BwE

Squrespace integrates with this one:

https://www.printful.com/integrations/squarespace

I'm having trouble getting Gelato to integrate with Squarespace.  Can you point me to info on how to do it?

I am trying out Fotomoto right now.  Seems flexible and easy to install and get running but having a few teething issues with the pop out sidebar for it being partially hidden behind the lightbox view of the image.