http://5rzkisd745el4yr7krh5qc4f5sgblvg4i5qtuxivfgxcxtr65ejd7gyd.onion/cart
Browsing works fully without it — but the cart, checkout, and order-status pages need it, because your shipping address is PGP-encrypted in your browser before anything is sent (the server only ever sees ciphertext). In Tor Browser: shield icon → set this site to Standard or Safer.