Guests pick half or full day and how many skiers. This one is an example we made.
Alpine Line lists half days and full days in the app. A guest picks the length and how many skiers, sees the price and sends it. This one is an example we made.

They quote or invoice from the same app. No web inbox.
Yes. List the base lesson and an extra-person add-on. The page adds them up.
No. They use the web page. Only the business needs the app.
Get the app. Add your lessons and group sizes. Make your page live and share the link.