when you place an order, they ask if you have comments. your comment should be "deliver in back". if that doesn't work, a discreet note on the front door should tell delivery men to deliver there after receiving no answer at the front door. delivery guys will usually oblige.

I once found a package days afer it had been delivered hidden in our bushes in the front yard. We never use the front door unless we have company. It was by the front door behind some shrubbery. It could have stayed there for a long time if we hadn't had someone ring the bell to sell us something!