Thanks so much for asking @Aim42! I'm Nora the shipping and fulfillment product manager at Squarespace.
To answer your question, currently Squarespace shows retail rates and ShipStation and Etsy show commercial plus rates - thus the difference in price. Notably, the rates vary package to package so the difference will not always be so large.
We're working to update our systems to extend commercial discounts to our merchants, but it's a notable undertaking involving contracts with USPS and major updates to our checkout flow. I really appreciate you raising this issue and know our number one goal is to help you get the cheapest shipping possible and extend that pricing to your customers. One workaround in the meantime is to offer flat rate shipping that's the average of your shipping costs.
Let me know if you have any questions, happy to discuss further on a call or follow up on any of these items and thank you for raising your concerns!