You may wish to add a discount to your treatments based on the amount that is purchased. You can do this by using volume pricing, e.g. 1 zone of botulinum toxin = £200, 2 zones of botulinum toxin = £360 (instead of £400). 

To create a treatment with volume pricing navigate to Menu > Record settings > Treatments. Create a new treatment, and fill in the treatment details as well as the price (including taxes). 

Next, select Volume pricing. Fill out the cost of the treatment once volume pricing comes to into play.