My client told me she handed her quarterly $3500 checks to the county Commissioner of Revenue, and they accepted them, gave her stamped receipts.  But the city Commissioner of Revenue refused anything over $2500. I don't think a certified check would be any different. They want it done electronically, period. But being a non-techie senior myself, it's daunting to sign up online; and my list of passwords and PINs is a mile long already. I personally ask that my RMD go totally to cover taxes, and try to nudge my clients somehow away from doing estimated taxes. They are a hassle. But this client has all her income from investments.