Ombrello is not a product of the usual kind. The unique formulation of Ombrello joins with your screen on a molecular basis to create a water and dirt rejecting barrier. Ombrello may be applied as often as necessary.
If I recall correctly the rear glass treatment was never available as an accessory, but was "supposed" to be reapplied every service and pretty sure is was about £20 on the bill. Needless to say few took up their "generous" offer and Audi stopped suggesting it shortly after.