Mileage allowance for trips to the post office as an expense?

I thought I could claim the kilometres I drive to the post office to do online orders or occasionally to a supplier to pick up goods myself as expenses for my company (I am self-employed in secondary occupation).
Is this possible with the flat-rate mileage allowance? And if yes, what do you need to keep track of? Do you have to keep ALL your fuel receipts? And how do you record something like that? Create your own document with a summary: date/destination/mileage … ? I’ve tried to search a lot of things, but I can’t see the wood for the trees 🙁