This
recent article says that you can mix money and points:
I cannot test because my points balance is zero, but what happens if you choose the pay with points option even though you do not have the full amount? It might offer a split payment option.
Here's what the final screen says:
Total
USD $678.94/67,894 miles
And right below this it says "Confirm and book."
Since this is a non-refundable ticket I don't want to take a chance and then end up with an e-credit of $678 with DL if I choose to cancel later. I rarely fly DL so I may never use the credit. I'll just wait and use my 48K pts on something else. Thanks for the link.