Accidental add-on

I accidentally added the early alpha access add-on to my Kickstarter Backer order, and now I cannot complete it. I don’t use PayPal so I can’t pay and I’m not interested in the early alpha after all, so I’d like to remove the add-on so I can complete it, but that doesn’t seem to be possible.

The Backer account uses the same e-mail as the forum account. Any help would be appreciated.