I think this argument boils down to how you price your servicing.

If you Q a price for a service inclusive of X,Y & Z and this includes a Forte flush, then I see no problem with this. However if it is not included in the price then advice should be sought from the customer before using such products. I would also agree with Daves suggestion of having a large point of sale notice adivising customers of the use of additives/flushes during the service.

Mike