@olifantoliver I said button, because that’s cheap to send worldwide.
I’d go for a sticker that can be shipped in a plain letter.
And I’d wish them not to ask for my address details before telling me that I’ll get a perk per snail mail. Yes, they likely need my address for tax reasons, but I’d much prefer telling my bank to send money to Mozilla’s IBAN than telling Mozilla my IBAN.
And then they’ll get my address for the perk.