It's when you don't have enough sats in your custodial wallet¹. Then your post is marked as freebie:

I didn't have 420 sats for ~mostly_harmless since I setup autowithdrawals such that any excess above 100 sats gets withdrawn to my LND node and you must have enabled free posts in your territory settings (checkbox below post cost). You can disable it, then everyone has to pay the full cost. It will show a QR code if they don't have enough sats in their custodial wallet.

Freebie items are only visible to people who have enabled greeter mode in their settings or if they got zapped. This is the help message for greeter mode:

UX around freebies with attached wallets is still WIP.

Maybe we should simply attempt to pay the full cost first before creating the item as a freebie. I would be happy to pay the ~mostly_harmless fee :)