Shipping costs for multiple items issue

Hello, I am setting up my Squarespace shop … and I am having a shipping issue.

If someone wants to purchase product A and product B, and product A costs $10 to ship and product B costs $21 to ship, I want to apply the highest rate of shipping ONLY, not add shipping for A and B together.

So I want it to be $21 in total, not $31 in total. How do I do this?
And if it's not possible, is there an extension that can handle it?

I am on the basic commerce plan.