Not a breeder but a puppy buyer here
There are lots of cute things that a breeder can send home and while they are nice (harness, leash, bed, etc.) they are not necessary.
Here are the things I look for:
1. a small baggy of the food you have been feeding... it is hard enough for the little guys to adjust, its best if there food can be kept the same for the first few days at least... recommending this also prevents the pup from getting an upset stomach and the new owner freaking out "omg this breeder sent me a sick pup!"
2. some literature on raising a yorkie puppy, either something you wrote or just some information you printed off the internet... how to help them adjust, how to start potty training, how to notice the signs and prevent hypoglycemia... hopefully your new puppy buyers have done all this research on their own but it is nice for them to have all the information in one place to readily access if they have not.
3. A list of things they should buy:
* crate
* expen
* Nutri Cal
* harness
* ID tags
If you find your puppy buyers before the pups are ready to go home, you can send this to them early so they have time to shop and are ready to go.
4. a small blanket or toy that the puppy and their momma has been using... this way when they are in the new scary place they have something that reminds them of "home"
Hope some of these things help, good luck!