Using debug enabler I found that every mailbox in the town is filled with glitched bills that cannot be interacted or moved and can only be deleteted with cheats. What is causing this? I have auto bills pay on. This may have impacts on gameplay, is there a way to purge mailbox inventory from times to times?

I encountered this just right now with my active lot.
I send my sim to get everything from the mailbox.
Really a lot gift certificates were there and some gifts went to family inventory.
I used Mastercontroller on my sim, Sort Inventory and these certificates were gone and I got the money from it.
Then I checked my family inventory and sold everything.
You can use MC - household -> sell items (inactives cannot use what's in there, do this after every town reset)