I would call right away. This happened to me. What I found out was that they did NOT automatically charge the rest of my package to my credit card, as I thought they were going to. I called at about 16 days out, like you, and they said, "Hmm, it's coming up on the computer as failure to make final payment." I was like, WHAT???? I'd missed the "deadline" to make the payment. Luckily they didn't care about the deadline, they were happy to have me on the phone to authorize final payment on the credit card on which I'd made the deposit. I was really shocked and surprised that they hadn't called or e-mailed to ask, or to alert me that the payment was due. I had been under the impression that my account would be automatically charged for the remainder, but they said, "Oh, no, you need to call and authorize."
Crazy. Anyway, it all worked out fine and I got my package a couple of days later.
Let us know what you find out!
Edit: You know, though, I just recalled I booked through Costco, so maybe that had something to do with the silliness.