I chose to do the deferral on my own return and received the IRS letter last night. The reply voucher was in the letter. Payment goes to Ogden UT 84201-0059.

Letter number is LTR3064C with a barcode for my SS# and a notation of "BODCD-NOBOD"   (I have not determined what that notation means.)  The reply voucher does NOT have any dollar amount even though the letter mentioned once the amount that I deferred and that it was due half at 12-31-21 and half at 12-31-22 as well as saying that I could choose otherwise.

This letter does not mention paying thru EFTPS even though the IRS press release 2021-96  (July 6, 2021) suggested EFTPS and other methods.