Signing in
"The username or password is wrong." Check both and try again; usernames are not case-sensitive, passwords are. "This username is taken. Try another one." Someone already uses that username: pick another made-up one.
"username must be 3–15 characters: letters, digits or _." The username has fewer than 3 or more than 15 characters, or a character other than a letter, a digit or _. A password must be 8 to 72 characters long.
Your session lasts 8 hours. After that the shop signs you out, and the API answers "The token is invalid or expired. Sign in again to get a new one." Sign in again; your cart and orders are still there.
Cart
"You can add up to 10 of one product." The cart already holds 10, or the new quantity would go over 10. "Only 3 left in stock." The shop has fewer than you asked for: lower the quantity. "Ostra Smart Kettle with App Control is out of stock." The product cannot be added until the next delivery.
"Your cart is empty. Add something before you place an order." Checkout needs at least one item in the cart. The same check stops a coupon on an empty cart.
Coupons
"There is no coupon SAVE30. Check the code and try again." The code does not exist. "Coupon WINTER10 expired on 28 Feb 2026." The coupon has expired. "Coupon SAVE25 is valid for carts of 1 to 10 items. Your cart has 11." Remove items until the cart has 10 or fewer. "Coupon SAVE25 is already applied. One coupon per cart." Nothing to do: the discount is already in the total.
Orders and payments
"Order SH-100012 is paid and cannot be cancelled." Only an order awaiting payment can be cancelled. "An order needs at least one item. Cancel the order instead." You tried to remove the last item of an order.
"The card was declined. Try another test card." and the other decline messages: the order waits in Orders as awaiting payment; pay it again with 4242 4242 4242 4242. "Only test cards work in this shop." A number that is not on the test cards list was refused before any payment.
"There is no order SH-100041 in your account." The order belongs to another account, or the number is mistyped. Nobody can open another account's order.
When the shop itself fails
"Something went wrong on the server. Quote the trace id when you report it." The message comes with a trace id, a string of letters and digits. Copy it into your report: it finds the request in the shop's logs.
"The shop did not answer. Check your connection and try again." The request did not reach the shop at all. Wait a moment and try again; nothing was saved twice.