To avoid overhead for many small payments, start a platform where users can buy many credits at once by contributing larger amounts to charity. Then, you burn your credits to apply to companies (or cold message applicants) to show you're not just spraying and praying.

Some more thoughts before I go to bed.

This could also be used for combating spam elsewhere, like posting in forums, comment sections and so on. To preserve privacy, something like zero-knowledge proofs could be utilized. I don't know how the cryptography would work exactly, but if you can't double spend a credit and you can choose whether to keep it anonymous or not, it could work, too. It would be best if for a given credit spent, you could only disclose your identity to the entity you want access to, not the credit issuing entity.

For spam, it seems like the cost of maintaining a forum like the servers are much lower than the cost of the mods that deal with spam. So instead of paying the forum directly, we lower the need for human mods to spend their time. That way we lower resources to the forum indirectly. The credits could be per post or per account creation. I assume the HN mods' time is worth a lot more than the servers and power HN runs on.

Also, we won't have the issue that PoW and other proofs-of-X's have of being easier to do on some devices, but harder on others (like the power and time it takes to run PoW on a beefy desktop with AES-NI vs an on old phone).

But we'll still have the issue with different standards of living in different places making the credits more or less expensive for the user subjectively. Companies hiring worldwide could require different amounts of credits for applicants from different countries, but for forums this wouldn't work.

A solution to that could be issuers giving credits for local volunteering work. Clean up some garbage from the shore and get a credit regardless of whether you're in the USA or Bangladesh. But if you want to prevent credits from being traded (do we? idk) and, at the same time, have some amount of privacy, how would you do it?

But now you'd have to make sure that credit issuers all over the world only issue credits for real charity-like work. And who's to say how to value picking up garbage vs volunteering at an animal shelter vs donating 1$ to a charity.

It's interesting to think about this, even though I don't have any resource to implement anything like that.