The services of Let's Encrypt, let alone the cause they stand for, is well worth a small recurring donation. I think this is totally reasonable to pledge even $2/mo.
If companies that uses Let's Encrypt would chip in just 1/10th of what they'd pay for certificate from for profit providers, then Let's Encrypt won't be in need of donation anytime soon.
That'd be a bit ironic, ICANN funding a project to verify assigned names and numbers because decades ago folks decided they couldn't trust ICANN to do its job.
