Brithunter,
When Post Office present the documentation to HMRC they have to pay any Duty and VAT at time of presentation. This comes from their Deferment Account. Same as any other Freight Forwarder. This Account has to be two times the maximum monthly amount they expect to use. This amount is held in escrow by their bank. HMRC can then call on this IF the end of month "bill" isn't paid. Post Office still have to pay HMRC every month. There is zero interest paid on the money in the Deferment Account. Now, allowing 20% minimum that any Company should be using as GP you get an idea as to the scale. Charging a minimum fee (as per retailers charging £1.50 for card transaction under £10) is cheap as you have to offset the loss of interest earned on the millions you have on deposit.
Now, if at point of delivery the client declines to accept the goods, Post Office can't reclaim what they have paid to HMRC. Same as any Import Broker.
Stan