Validation Error Codes

Code	When to use
`product_not_found`	PLU does not exist or is not sold at this location.
`product_snoozed`	Product exists but is temporarily unavailable (86 / snooze). Also inferred when `available: false` and available quantity is `0` or omitted.
`quantity_unavailable`	Requested quantity exceeds available stock. Inferred when `available: false` and quantity is greater than `0` but less than requested.
`price_mismatch`	POS unit price differs from the request by more than 1 cent. Inferred when `items[].price` is returned without an explicit error.
`tax_mismatch`	POS order tax differs from `taxTotal` / `charges.tax` by more than 1 cent. Inferred when totals are returned without an explicit error.
`unknown`	Any other validation failure, or an unrecognized code string.