Extras are optional in BRM / RSM.
Generally for things like pedals, helmets, insurance etc etc.
You set them up in your back-office to provide checkboxes in your reservations for tracking who gets what optional extras.
(you can either charge for these extras, or they can be free)
N.B. extras do not count towards your inventory count (BRM subscription)
however extras do not track your inventory - so you need to have 'an unlimited supply'
Extras on your online booking
The cool thing is that your extras can also be specified by your riders directly when they book online:
(and even better, you can specify which extras are applicable for which product lines - known as extras compatibility)
The choices your customers make feed automatically through, on each reservation as choices per bike.
collecting per rider information (like weight, height or ski ability etc)