I found out something else today. Those little Express Post Office stations don't stock flat rate boxes in the small size. I didn't have any because my online request for supplies never came in. So I hauled all my shipping stuff down there, polystyrene cut up into measured panels, packing material, heat packs, moss bundles, labels. The customer service person gave me medium size boxes, saying I could just use those. Yeah.. for twice the price!! I didn't have insulation sheets that would fit those anyway.
Very frustrating. I don't have a working printer, or I'd do all this online and schedule a pickup. That's another thing about the upsp.com site.. it lets you print the labels and postage to your own printer. And you can schedule your mail carrier to just pick them all up. This doesn't work affixing postage to the boxes if they weigh more than 12 oz.