Adding items
Sign in to add items: the cart belongs to your account. You can add up to 10 of one product: "You can add up to 10 of one product." You cannot add more than the shop has in stock: "Only 4 left in stock." A product that is out of stock cannot be added at all.
Change quantities in the cart; the totals update as you go. Everything you add stays in the cart until you place an order, remove it, or reset your data.
The summary
The cart's summary shows the subtotal, the coupon's discount if there is one, shipping and the total. Prices are in USD; the currency switcher only changes how amounts are shown.
The total is the subtotal minus the discount plus shipping. The discount is never more than the subtotal, so the total is never below the shipping fee.
Checkout
Checkout has two parts. Delivery: Recipient name, Street address, City, Postcode and Country. Payment: Card number, Expiry month, Expiry year, CVC and Name on card. "Fill with example details" fills the delivery fields with a made-up address.
When you place the order, it is saved as awaiting payment, the cart is emptied, and the payment is tried with the card you entered. A declined card leaves the order in Orders for another try.