Lisa Keeble of ContractorUmbrella responds:
The question of whether or not the cost of postage can be claimed has the same rationale as any other expense; you will have had to pay to send your documents to your umbrella company because you have undertaken the contract and for no other reason. Therefore the requirement for ‘wholly and exclusively’ is met and the cost is allowable as an expense.
With regard to the mileage claim for travel to the post office, in theory yes it would be allowable. However, you would have to prove that there is no duality of purpose so you wouldn’t be able to post anything else at the same time as you posted your ID and, if you had a post office that was in walking distance but you chose to drive HMRC may question it.