technically they are correct; smaller packages (i.e., letters) do not attract the carrier's attention, so they don't always scan them and the post office doesn't want to be bothered with monitoring the complaints. But I use bubble mailers - and pay the box shipping price - all the time. That means the post office gets about $1.75 instead of 64 cents. If they fail to scan the item upon delivery, so be it.

I never take those items to the counter, though, so there is no debate. I just drop them in the slot. If the post office wants to reject the package, I'll cancel the postage (I use PayPal) and either re-send in a larger package or save some money and send it the normal way (first class + 20 cents non-machinable surcharge).