Okay! Heard back from the Backerkit team!
If you are struggling to add on the Cerberanian pin, these are the steps they say to take:

Click "Edit Address" in the upper right hand corner of their survey → Edit Add-ons to get to the add-on page. 

This is he direct link to the Add-ons page: https://paragon-tarot-pins.backerkit.com/backer/addons?editing=true

If thaat doesn't work, let us know so they can look into it further for us.