EDIT:   When you're client makes his payment To CT online he should select that it's an extension payment.  In Proseries just fill in the CT extension as if it's a mail in voucher.  Should be no need to efile an extension that;'s being made by the TP

Not sure but If you Efile the Ct extension it looks like they really want the ACH to accompany....

If you do ACH the money w/ the extension you have to complete the license info on federal ID verification to make it happen...  several states seem to require it.