SquareSpace is only integrated to use Stripe a credit card processing company.. Where you see "Add to Cart" this will use credit cards. You can use it to check out more than one product.
I prefer people use Paypal and you can find a paypal button at the very bottom of each products page. it says "Buy Now" and takes you straight to paypal. However, you can only check out one product at a time.. because Paypal is not part of Squarespaces set up YET they say.. So if you check out more than one item with paypal I will reimburse you the excess shipping amount you pay. You can always tell me which products you want if you are not worried someone else will swipe the item up by the time I get the message.. in this case I can send you a combined invoice straight from paypal.. I will just need your email address for paypal to do that.