SusanY1: Thanks again for the great response. As it turns out we tried a slightly different approach than you suggested. Hopefully it will work properly. First, the amount of taxes estimated for the newlywed couple's federal extension was paid via IRS Direct Pay using our adult son's name and SSN (no place to enter his new wife's name or SSN as mentioned in the OP) and a direct EFT debit from his bank account. Second, a nominal amount was paid via ACI Payments Inc. (one of the two IRS approved vendors) using PayPal. The ACI portal had a place to enter the name and SSN for BOTH our son and his new wife unlike IRS Pay Direct. ACI also takes credit and debit cards, Venmo, etc. The fee for using a credit card might be significant but the fee for using a debit card or PayPal was nominal, cheaper than writing a check and sending a Form 4868 via certified mail. Seems like paying the entire amount via ACI (or its competitor Pay1040) might be a viable approach for other newlywed couples assuming the amount owed is within their bank's debit card limits. Thanks again.
