Well, that was the easiest, yet most expensive, debit payment I've ever initiated.
I just got an email from Tesla with the finalized financial numbers, and a handy link to "Pay Balance."
When you click the link (which I won't show here, due to obvious privacy concerns), you're presented with a few simple informative line-items, and a down-payment amount. In my case, it also provided the financing principal amount (since I'm not filthy rich).
On that screen, you then click the "Make Payment" button; and after entering my electronic check info (just the usual routing, account, address, etc.), I promptly got a thank-you email.
Very very easy.
Just look at the time-stamps on those emails for proof of how quick it was.
Supposedly, according to my DS, all I have to do now is sign a few papers when I pick up the car, and I'm on my way.