I am just curious do you have a contract with your puppies? If not I think I'd work one up bc to be honest it sounds like your getting taken advantage of from people.
I agree with JL, she makes some great points. I think before purchasing the dog the potential owner needs to know the costs of taking care of it. They should do some research and I'm sure they'll have plenty of questions for you. I think the usual necessities for caring for a dog is the owner's responsibity not yours. Sounds like you've been more then generous with your buyers.
I think most people want the same things. A healthy puppy and the breeder to be available if they need them, not financially though
Hope you figure something out. Best wishes!