If none of the other alternative payment methods are suitable for you, the last possibility is to mail the payment fee. Checks or International money orders can't unfortunately be accepted, for the reason that their processing fees are higher than the actual payment (cashing a check or money order would cost more than the registration payment is), thus sending cash in a letter is the last choice.
Please notice that sending cash in a letter is insecure, but if you feel you're willing to take the risk, please send e-mail to address <oparviai@iki.fi> for more information.