I've seen prominent RVers recommended Amazon Prime, and I used to be an ardent fan too. When we lived in a house, we'd have Amazon packages with single items show up regularly. I loved having the motion-detecting security camera notify me of packages before the Amazon notification came through. (I wonder how much they paid for shipping things to me). When we hit the road, we needed to pay more attention to when and where we were ordering things, but we still loved the service.
However, that all changed when we upgraded from dying batteries and relying on RV park shore power to overkill batteries and lots solar. We started looking primarily for boondocking spots. We now consider commercial RV parks a "pit stop", and rarely stay more than two nights. That's not enough to comfortably coordinate package delivery.
When not boondocking, we often stay at NPS, USFS or regional campgrounds that may have a staffed entrance booth or (more typically) a camp host or patrolling ranger enforcing self-pay, but they definitely don't have someone to act as your concierge.
We often just dump and get water at rest areas, gas stations, or campgrounds/RV parks (without staying there), and move directly from boondocking spot to boondocking spot.
In small towns, we've seen RV parks where USPS does not deliver to their street address, so you need to give the street address to UPS/FedEx and PO box to USPS -- but you don't know what shipping method Amazon is going to use. Entering the PO box address will force Amazon to use USPS, but that's often slower and more unpredictable than UPS, and Amazon refuses to use it for some items (too heavy or large, contains lithium batteries, etc). If you enter the street address, you might get lucky and Amazon picks UPS, but they might as well mail your package only to have USPS return it as invalid address.
Sometimes, a UPS driver will go out of their way, call you and make an effort to find you, even driving down little forest roads looking for you. More often, you'll drive an hour or more to their shipping center to pick up your package, the next business day.
Getting packages delivered to where we were was just not working out.
Sometimes in larger cities there's an Amazon Locker within 30 min drive, and we use that. Except those can't receive large or long items.
Sometimes, a UPS office will also let you also receive USPS mail there, making it possible to use those no matter what shipping method Amazon picks. Last one we saw charged $5 for the convenience. Quartzsite has Quiet Times that offers a similar service.
For most of the last year, we've relied on the trick from Technomadia addressing Amazon packages to "PO BOX general delivery", which makes Amazon's computers to pick USPS. General delivery is a mail service where you pick up your mail at the post office. Just using "general delivery" in the address isn't enough, Amazon will ship via UPS for those, and that won't work. Technically, having the PO box there makes it an invalid address, but it gets delivered fine.
General delivery never worked for the items that were too large, long or heavy, where Amazon didn't want to pay USPS shipping prices, and just plain old refused to ship to that address. USPS also prohibits mailing lithium batteries and such.
But now, in the last two months or so, I've seen a new trend emerge on Amazon: they'll just flat out refuse to ship certain things via USPS, without a clear reason. Arbitrary small, lightweight, non-hazardous things now get refused for USPS-only addresses.
All I can think of is that Amazon is cutting costs as much as it can, and trying to move as much to its own delivery services and UPS as possible. Perhaps their deal with USPS is less beneficial to them, now?
All I know is, I can still find good deals on Amazon, but I can't buy them!
This has frustrated me enough that I've canceled my Amazon Prime subscription. If we're forced to very carefully coordinate when we get things from Amazon, at that point our orders are over the $25 free shipping minimum anyway.