Yes, when a player cancels. Additional items are always refunded in full, even if your cancellation rules only give a partial refund, or no refund, for the booking. The stock is returned too.
Example
A player books a court for £40 and adds a rental racket for £5.
Your rules give a 50% refund for this cancellation.
The player gets £20 back for the court and £5 for the racket: £25 in total.
Automatic cancellations
If a match or activity is cancelled automatically, the player gets a full refund, including items.
When you cancel a booking as an admin
You choose the refund for each player in the cancel dialog. Full refund is pre-selected for every player who can be refunded, so check each player's choice before confirming.
Choose Full refund if the player should get the items back too. With No refund, the items are normally not refunded either. With Partial refund, the player gets only the amount you choose.
Stock
When a booking is cancelled, the items' stock is returned, whatever the Refill inventory after session end time setting is.
Good to know
The cancellation email tells the player that the additional items they added have also been cancelled and refunded.