That's cool that you want to support. If you are able to do everything you want to do here then your presence is your support :-) .
I'm not sure if it is a viable option but you might email @membership and request a PayPal invoice. To pay as a guest through PayPal you would need to receive a PayPal invoice. I have no idea if this is a thing SDF does.
I signed up for a SDF/Freeshell account about 20 years ago. My donation was $50 in a birthday card sent from overseas. Somehow it made it there.
Can you buy cryptocurrency? SDF has been known to accept that from time to time. Run the 'dues' command on the system, or contact membership.
I am not sure how crypto works yet, but I'll check on that.
By mail: This seemed like a possible option. However I found that Slovak Post forbids sending banknotes or coins of any value via regular mail (for which shipping would cost €1.60) and instead only allows them in insured packages in which case the shipping would be €20.50 💀
I'm not aware of Slovakian law. But every other postal service in western Europe bans sending cash by mail for insurance reason. They don't want you to ask for the money back if it doesn't reach the destination. But they actually don't track down people sending money. I've sent cash by mail many times without any issue. Hide it in a folded A4 paper, so that it's not visible when looking through the envelope, and it should be fine, it has a 99.99% of arriving like any other letter.
That's my 2 cents.
You might be able to mail them a check. I don't know how banks handle a check from another country, so maybe that won't work.
Fans of SDF