It appears that there is an error with our store host and/or payment processor. I have submitted a ticket. My guess is that this is a black Friday-related error, as it was working just fine yesterday (after I corrected the error that JD3 was receiving).
I just went all the way through the order process and verified that we ARE receiving the orders, the confirmation page just doesn't show up. You should still receive a confirmation email, however. I am confident that this error will get fixed quickly, as Squarespace is a major store hosting service and I'm certain we aren't the only ones experiencing this.