The whole idea to check the donations came from stumbling upon this post which discussed costs per user.

Things should be put into perspective. The cost per user is actually the fixed monthly cost of operating an instance divided by the average number of active users.

In the discussion you linked to, there’s a post on how Lemmy.ml costs $80/month + domain name to serve ~2.4k users. If we went through opex/users metric, needlessly expensive setups with low participation would be a justification to ask for more donations.

Regardless, this is a good reminder that anyone can self-host their own Lemmy instance. Some Lemmy self-host posts go as far as to claim a Lemmy instance can be run on a $5/month virtual private server from the likes of scaleway.

Is there something else I’m not seeing?

Possibly payment processing fees. Some banks/payment institutions charge you for a payment.

Duplicate code can be a code smell, but it’s far better to have the same function definition or code block appear twice in the code than extracting a function that tightly couples two components that should not be coupled at all.

See Write Everything Twice (WET) principle.