We ship between 50 and 120 packages a day via the postoffice. I believe we do put in a request to do so and you can only schedule a month or two in advance but then you reschedule. I do not handle the schedule anymore but I recall it was a pain to get started - nobody knows anything at the local level.
She picks them when she comes by to deliver mail and will stop even if there is no delivery for us. She has even stopped on Saturday when we have no mail to do pickup.
All of this for free! Take advantage of it while you can though, the post office is bleeding red and I believe they will get there act together at some point. I for one hope they succeed, because UPS and FED-ex will charge too much for all the little packages we have to ship.