I just noticed the extended dates on the website. I plugged in my dates on the website to check and see how much of a discount was available. I could save ~$36 on a full hook-up. I called to have the discount applied to my October visit. The agent asked for the code on the site (AK9) and tried to apply them to my dates.
I told him I had the browser window open and showing the discounted price for my travel dates. He asked if I had searched for the exact dates of my visit. I told him no, I had to search with extra dates for my exact dates to show up. The agent said my exact dates weren't available for the discount and asked would I like to change my dates or book a cabin.
Right now, I can book a new reservation with the discount (adding two extra days to get a full hook-up to show up). I'm going to book a new reservation, call and cancel the two extra days, and then cancel my previous reservation. It's a lot of work for $36 but that can help pay for my HDDR tickets!
I suggest you book everything online. I never call DRC unless absolutely necessary. You have the benefit of searching for extra days online, booking them, and then immediately calling DRC to cancel the extra days. When booking online, click "Save at least 25%" under the picture on the WDW main site and follow the prompts to book.