What does this phrase sound like to you? Weigh and pay at the cashier.
To me, it sounds like this: You can take the stuff of your choice, weigh and price it at the rate indicated and make payment to the cashier.
In this case I would add at least an apostrophe since ‘cashier’ is a person: Pay at the cashier’s.
Yes, I support your suggestion even though the meaning is clear otherwise.