Start with the general rule

Airbnb prohibits moving bookings outside the platform, including offering discounts to do so. Its policy also generally prohibits collecting reservation fees externally and requires mandatory fees to be disclosed in the proper pricing fields.

Sources[1] Airbnb

Then check the exceptions

Disclosed local taxes, certain software-connected hosts and hotel procedures can qualify for exceptions. Hotels may collect some optional fees outside Airbnb. Ordinary hosts’ optional add-ons generally go through the Resolution Center.

Sources[2] Airbnb

Ask for the exact line item

Compare the request with the listing and checkout breakdown you saw when booking. Ask support through Airbnb if the fee or exception is unclear before paying. Keep the conversation and receipt together.