Calendars need to have an option to customize service bookings at different rates based on add-on services/products selected.

For example, using MassageBook, my client is currently able to offer a calendar to schedule massage services at a base price, then as they are scheduling they can select additional add-on services (hot stones, essential oils, vibration therapy, etc.) each of which has an additional price that adds on to the total price that they book the appointment for.