Just Donated to Sierra Club

Just donated my $15 and didn't realize cancelling would be a hassle. After reading through some old threads, I have to donate for several months otherwise I may not receive my SBs?

That's sort of ridiculous since the disclaimer only says $15 minimum. It also hasn't even shown up on as pending just yet.

I guess worse case scenario is I'll charge back to my credit card.

Anyone with experience on this offer?