A few years ago, I implemented a https://en.wikipedia.org/wiki/Delay-tolerant_networking project.
This is not exactly mainstream, although not ridiculously esoteric.
At the time, we used $200 embedded microprocessors, but now I would presume that $10 Raspberry Pi Zero Ws would do the job.
Having Googled around, it seems that https://github.com/ibrdtn/ibrdtn/wiki/Install-IBR-DTN#debian-arm is the solution.
However, before committing myself, I want to ask those who are actually developing for DTN on the Pi, in case there is a better alternative.
I am also looking for a low energy solution, as battery life is a factor.
[Update] Obviously, I would word any question with some exact requirements, so as not to make it a "what's best question"