it’s number 1 on the list of addon TOS, no paid wall at all is allowed.
Add-ons must be free of charge.
All add-ons must be distributed free of charge. Developers may not create “premium” versions of add-ons with additional for-pay features, charge money to download an add-on, charge for services related to the add-on, or otherwise require some form of monetary compensation to download or access an add-on.