If you want to avoid having any backup devices yourself, would you need to upload to two or more Glacier-like services?
I also trust Amazon to stay in business but mistakes can always happen (not just hardware but ID theft, card expiry while you're on a 6 month hike, etc) and unless you're prepared to say "losing these is OK" using one service would be risky.
Offsite tape backup can approach that price, but I don't know of any exact alternatives. It would certainly fulfill the requirement of having two different media types and providers.
I also trust Amazon to stay in business but mistakes can always happen (not just hardware but ID theft, card expiry while you're on a 6 month hike, etc) and unless you're prepared to say "losing these is OK" using one service would be risky.