You can choose whether you want to allow flexibility for users to create extras when managing bookings or require the user to select from the defined list of extras only. To do this, any user with full access can toggle the appropriate settings as follows:
Go to Settings > Extras > Select the option you prefer for your account
Flexible: Allows users to create extras when managing bookings
Select from list: Requires user to select from the defined list of extras only
The setting will be applied to the whole account for all users.