Does this sound familiar?
"I can't pre-allocate a bike coz I have no idea which bike I'll actually give the customer on the day".
Imagine you have 25 medium hybrids. You can't pre-allocate bike HM23 coz on the day you might end up giving the customer HM2 coz it happens to be at the front of the pile of your medium bikes. No one will ever move 20 bikes out of the way to get to the one on the reservation.
For this reason BRM invented 'GBAL mode' - Generic Booking Allocate Later.
make a booking for a medium hybrid and allocate the actual one they walk out the door with - only at the time of pickup.
(without fear of over-booking your medium hybrids - best of both worlds)