Hi Sherri
If this is a rare occurrence, then enter the ACH as a manual check, with the "Print check" box unchecked, and enter a fake check number (such as the date). If the invoice has already been posted to the vendor's account, then use a Payment type of "Payment", if not then use "Cash Invoice".
If you are going to be paying most of your vendors by ACH, then there is a different answer for that. Since you are in the US, it is most likely not going to be Adagio EFT.
Steve