If I email a breeder and they start making request like that I will personally pass simply because what is to say that the way I keep my animals will be to their liking and do I want to jump through hoops afterward and explain the way I keep my animals.
After you do this a while you will get a feel for people soon as you start communicating with them without having to be nosy and making demands.
Now if this is something you really want to do I could see 10% percent but not $10, if the animal purchased is $250 or more it will not mean much to get $10 off.