you should have a service type item for the camera, rate is 50
use that item on an invoice or sales receipt, qty is number of days rented
use it again for the second camera
or if you usually rent two at one time, create an item for two cameras with a rate of 100
or you could use a bundle item
list the camera item, and a qty of two
then on the invoice or sales receipt for the bundle qty enter the number of days rented