If your Renter asks you to add add-ons to their confirmed booking and you can provide such add-ons, go to Bookings > the booking in question, and at the bottom of the page click “Update Booking”:
On the page that will open up, you can add add-ons to the booking. The pricing on the booking details will be adjusted accordingly.
Your Renter will receive a notification in-app and also to their email informing them of the update pending their approval. Once approved, the Renter will be charged for the add-on and you will be notified in-app and via email that the update has been completed.