It's arguable since it's "donation" so one may expect to get the full donation instead of 30% cut by google but one may argue what exactly is (or isn't) a donation.
I’m not sure why you’d expect to not pay a fee. Google’s expenses are the same for apps that take a charge as those that take a donation. Credit card processors charge the same fees on donations.
A donation doesn’t magically make everyone else work for free.
> Credit card processors charge the same fees on donations.
Yes, they charge 1/10 of what Google charges. The 30% has nothing to do with how much it cost them to run the service, so let's not pretend that they _have_ to charge them.
(I'm not saying they should offer cheap/free donations, I just don't think it's fair to talk about how much it costs them to run the service given that that's clearly not the driving factor)
A donation doesn’t magically make everyone else work for free.