Just change your pledge amount by the amount of the add-on and you should be good to go.
Important note: do NOT change your reward tier! For example: If you're at the $30 tier and you want a lighter, go in and up your pledge by $45 (to $75) but don't change your reward tier- if you change the reward tier to $75 you get more coins, but no lighter. When the project closes we will be sending out a survey- THAT is where you will indicate which add-on you wanted. (Also which coins you want and where to ship them to- that sort of thing.)