Hello there, @fpbinvestor123.
Thanks for reaching out to us. Allow me to add to this discussion and provide some clarification about sending an invoice with eCheck as a payment method in QuickBooks Online.
As per checking the screenshot you provided, you have successfully followed the steps given by @MarshallA. The Free bank transfer option you’re seeing on your invoice includes both eCheck and ACH payment methods.
You need to enter your customer's bank information and get their authorization to process payments. See Process ACH bank transfers right in QuickBooks for your reference.
That should do it. Please touch base with me here if you have any other questions. I’ll be more than happy to help. Take care and enjoy your day!