We use fetch as our word so when I started I use to go to the door with Harv and say fetch it and he would take it out, I would then swap him mail for a treat. When we are not home we shut Harv out of the kitchen so he can't get to mail. Now he tells me when the postman arrives like any typical show cocker so I can call out fetch to him and go and collect (mean mum no longer gives a treat, thankfully he is happy with a bit of praise). My aunt and uncle trained this with gate deliveries as they live rural and they have a gate at the box so can send their cocker out on command and recall in exchange for a treat so it is possible inside and out. The only think I have yet to master is the mail that lands on the floor...
Good luck, it is a good one to train as a positive....