It appears that somewhere in the meantime the PayPal finally changed its policy related to Polish customers and allowed me to receive donations. I've added the button to the download page, and at the same time I've removed the AdSense ads from the main pages (they were of very poor quality recently), while keeping them on the message board.

Thanks for setting this up Thomasz. I feel much better now having the ability to show my appreciation for your efforts on our behalf.

What was the original policy?

Many countries are only allowed to pay, not to accept payments. This is case for Slovakia, for example.