YOU SHOULD ALWAYS SEND LETTERS WITH 'USPS RETURN RECEIPT REQUESTED.' (It's the green card you get at the post office and once the recipient receives your letter, you will get a green card in the mail serving as proof of delivery and acceptance.) This is the lowest costing form of legal proof for a paper trail. Well worth it. I use it all the time and I learned this from my dad who has spent his life practicing law.