So SORRY but YES Kickstarter has a bug and they haven't fixed this yet. There are 2 ways to work around this and make it work:
(1) Contact me via Kickstarter Messages and explain which packages you want to combine and we can create a special one for you,
(2) Buy the most expensive reward package, but add the extra cost of the additional reward package(s) that you want to this package. Then send a message via Kickstarter to let me know what the extra adjustment is for? I will then send you a confirmation of what you'll be getting.
Thanks again for your interest in multiple reward levels.