I put this under 'Entertainment' because I never fail to be entertained (every single time!) by the sales staff's expressions when I politely refuse their request for my postcode!
I simply say to them, "That's OK, it's just a cash sale." Even after stating this, some have persisted and said that it's "only in case you need to claim a refund" or it's "just for the invoice." A refund or invoice for a bottle of wood glue or a box of screws???
I'm never asked for my postcode whenever I buy a Snickers bar in the supermarket! GDPR, folks.