To create an 'Extras' page for customers to add additional items to their basket when making a booking e.g Club Rental, you can follow below steps.
Settings > Flags > create a Flag for your group of Extras e.g. 'Club Rental'
Settings > Products > create your Products
Settings > Booking Extras > create your Extras Group and add your products to it.
You will now have to add these Extra Groups to the Bay Options and Course Options using Flags.
Settings > Booking Extras > open Group, add Flag
Settings > Course Options > open Course Option > add Flag.
Settings > Bay Options > add Flag.
Refresh the customer booking page, make a test booking and you will see your Extras on the customer booking flow.