In my days we were sending them money via Paypal and if it was above a certain amout, then they'd kindly ask for your postal address, draw you a sketch (I have three, one of them with two dwarves in minecart jumping over a ravine, one of them has a beer in his hand) along with a button.

Donations. They have a patreon and have had paypal donations long before that with the occasional donation drive to pump up numbers.

Yeah they used to have a PO Box you could mail checks/etc to. I still have it in my billpay. The only gaming value that has come even close for me has been Factorio.

Maybe aurora 4x could be to your liking.

Probably from their Patreon: https://www.patreon.com/bay12games