No. Discount codes only reduce the booking price. Additional items are always charged at their full price.
Example
A player books a court for £40 and adds a tube of balls for £6.
They use a 50% discount code.
They pay £20 for the court and £6 for the balls: £26 in total.
Good to know
To charge certain players less for an item, give their cohort its own price on the item in Shop → Inventory.