Cart mandate
A cart mandate is the record of what a shopper authorised an agent to buy: the specific items, the total, and the conditions attached, captured in a form the merchant and the payment provider can check without taking the agent’s word for it.
It exists because an agent purchase separates the person from the transaction. In an ordinary checkout the authorisation and the order are the same event: someone looked at a total and pressed a button. When software presses the button later, something has to carry the shopper’s consent forward and bound it.
The bounding is the point. A mandate that says "buy this, at this price, from this merchant" is checkable. A mandate that says "buy what you think best" is a blank cheque, and no payment network is going to underwrite one.
Why it matters to a store
It is also why price accuracy stops being cosmetic. If the total at checkout does not match what the agent was authorised to spend, the purchase fails at the last step, and the failure looks like your fault to the shopper.