runs the cleaner and then tells me to BUY it...
Remove the Cleaner component. It's not mandatory to be protected from any viruses or threats, and such kinds of auto-cleaners are notorious in ANY software, not just Avast, for being annoying, slowing down computers, and sometimes removing what you actually didn't ask it to.
Plus, some cleaners can break your system registry, if they also clean it up too. So never use any cleaners on your computer if you don't want to break it, in general.
And for Avast, yes, the cleaner is unnecessary and annoying. I never used it, and I'd recommend you to avoid using it as well, considering it causes so many problems for you anyways. Just remove the component and you'll be fine.