It could done behind the scenes, although I don't know how much space a typical post office has for storing mail for an extra day or five. They could hold off

It could done behind the scenes, although I don't know how much space a typical post office has for storing mail for an extra day or five. They could hold off delivering to a group of addresses (a street or an apartment building) until they either have a priority letter, for which a premium has been charged, or a non-priority letter is reaching the deadline, which here is 5 days. At that point they deliver everything that's waiting.