This is another unfortunate example of things that are problematic and/or not logical about Squarespace commerce. My client wants her customers to agree to terms in order to complete the purchase. I was only able to add a required yes/no to the agreement at checkout, but if someone clicks "no," they can still check out.
Also, the store policies are not visible at checkout until *after* a person puts in their payment information. They need to be visible on the checkout page at all times, especially because the links to those things from the actual website (i.e., in the footer) are not accessible from the checkout page.
I've read many recommendations that if you are planning a larger store, definitely don't use Squarespace. But I've so far only worked on sites with very limited offerings, and I'm still always stumbling onto needed features that are absent or deeply flawed.