After you sign a petition, you're offered the chance to pay Change.org to advertise the petition to a wider audience. I have only ever used this "chip in" feature but was billed for a monthly membership program. Disputing this charge with PayPal had no effect. The change.org website allows you to cancel the membership program, so be sure to check if you were enrolled in the program after chipping in and cancel if you didn't intend to do so.