Answer:
Cash
Step-by-step explanation:
As an owner of a small sandwich shop, the least costly way a customer can pay you is by cash. There is no further information needed about the customer and no additional fees are charged to to you. The only thing to be aware of is that the money is not counterfeited.
When someone pays with check you will not know for sure if they have the money in their account. If they do not, this can be a costly process for you if you wish to obtain the money for your sandwiches.
While you are guaranteed the payment with a credit card since it is paid by the bank, you will need to pay a fee to them for this guaranteed payment.
A debit card is basically like paying with cash, but the transfer is not instantaneous and it's possible the customer could leave before you realize he or she did not actually have the funds in their account.