Paid features, consumables, or locked content need visible price or purchase indicators in the app's UI — burying the fact that something requires payment until the user is deep into a flow isn't allowed.
Frequently caught when a "free" app's core loop quietly gates progress behind an unmarked purchase a few screens in.
"Your app offers in-app purchases that are not clearly indicated to the user prior to being presented, per guideline 2.3.2."