SDF even accepts cash via mail, including EUR. Then the problem is mailing money from Slovakia. Slovak Post doesn't allow money of any value in regular mail sent abroad. And the alternative is too expensive.
I don't have a problem donating money via PayPal. I did so with KDE, Linux Mint and some others. But guest payments to SDF don't work from Slovakia, just like with Videolan. But Videolan at least allows sending money directly to their account.