When I used to use a Nationwide FlexOne account for banking I couldn’t use my card in my local post office to buy anything because they’re unable to do cash withdrawals on that card.
I asked the cashier, and he said ‘when you make a card payment at a post office we make a cash withdrawal from your account to pay for the transaction’
TL;DR - if you can’t make cash withdrawals, you can’t pay with your card
This may have changed recently, but I always carry change to the post office because of this.
EDIT: What @chalky said.