Where is DHL that is asking for proof of identity? Why can’t DHL UK help, surely they would be delivering, I do not know if USPS actually deliver themselves in the UK, more likely to pass to a UK delivery company.
What have you bought, is it something that requires proof of Age before delivery?
Google says USPS does not deliver in the UK but uses Royal Mail.
Best to wait and if not delivered by expected delivery date, open a NOT RECEIVED case.