I have been asked to reply.
Customers are not charged when they use their Post Office card account or cash a DWP cheque at the Post Office. However, the contracts for these two services come at considerable cost to the taxpayer. The Post Office card account contract, for example, will cost up to £1 billion over its lifetime. Each payment into a Post Office card account costs around 80p and each cheque cashed around £1.79. This compares to approximately 1p for each payment into a bank or building society account.