<prompt> Human: I need to owe some money to IRS and planning to send that as check to them . What is the best way to dispatch the mail ? Fedex 2 day delivery or USPS ? . Please share your thoughts. At the end of the day I dont want IRS to come back to me and say that they have not received my check. I wanted to have a proof of posting and proof of delivery. Thanks  <response>: I would pay online; you get a record immediately. Otherwise for posting important documents, I usually use USPS priority mail its fast, cost effective, and tracked so you can verify delivery. Avoid services that require signature on delivery, as apt to slow things down. 