If you want to rely mostly on maildrops, that's cool - it's what you want. One thing to keep in mind is you said you didn't want to rely on hostels, motels, etc because of keeping your costs down. But if you send all your maildrops to POs (as opposed to hostels & motels which accept them), you'll inevitably at least once find yourself arriving in town Saturday afternoon and having to wait until Monday for the PO to open. And some POs are closed totally on Saturday or have limited and odd hours.
You may find yourself hiking all night to make a Saturday maildrop. Something to think about.