Regular first class mail with no bells or whistles.
Registered, Certified, Signature Confirmation, etc. will not help and will only cause delays. In short, you pay more to get a delay. Priority is unecessary too b/c since you are mailing regionally, chances are that it will be received the same day anyway.
This is not a filing that you may need to prove it was timely made. Any receipt will do no good if it is misplaced, and you'll get real confirmation whne they send the receipt and cash the check.... if they don't, ask yourself: what are you going to do? Refiling or trying to argue the USCIS with your USPS receipts in hand? Refiling of course.