Sending e-mail to info at lemmy dot world would be the best option. The team will pick that up.
(By the way: 2 EUR per year would be enough if every active user would donate that but they don't)
For lemmy.world / mastodon.world including some of the smaller instances we host, it's like 0,06 euro per active user per month. If every active user would pay 2 Euro per year that would be enough to cover hosting costs. If every active user would pay 1 euro per month, I could quit my dayjob and focus on the Fediverse fulltime.
(Sidenote: Stux and I created the non-profit Fedihosting Foundation which owns lemmy.world .. but finances are still separate for his and my instances)
We have done before, but don't do that often, so we don't bother users too much. But we might do again soon, and maybe do a monthly post on the mastodons/sharkeys etc