There's many places that sell a certain number of days, plus 1 day free. If you compare that price to other vendors who are just selling ticket days with no gimmicks, you will find little-to-no difference in price. It's just a marketing ploy to entice people, and see, in your case, it appealed to you. You have to compare what Disney tickets cost among a few vendors, to truly know if you are getting a "free" ticket day. When comparing prices, some already include tax and shipping, other sites post prices NOT including tax and shipping, and that makes their tickets seem cheap. Some places hide the shipping cost to the very end of the transaction, then you suddenly find out shipping's around $20 (should be less than $1), wiping out any savings you thought you were going to get. Tread very carefully and look for hidden tax and shipping charges.
Best bet for best price,
Undercover Tourist. If you have time, sign up for the Mousesavers Newsletter link for a few dollar additional discount.
UT does do the "free" day thing on some tickets, but their prices are still the best, their service is honest, reliable and fast, and people on these boards swear by them year-after-year, myself included. Their prices already include tax and free shipping, nothing hidden.
I understand your idea to Park Hop but it won't work. We always get hoppers, or plan to stay a little longer. Can you stay one more day?