What I recommend you do is place your order without the DTH, and in the comments section request that they add it to your order and state that you are aware it won't be in stock for another week. This will delay your order until it arrives (Approx. next Thursday), but this way it will be shipped for free.
There's a slight chance if they're having a really good day they might ship it for free like you requested afterwards, but that would be an exception and not the standard. The free shipping is a discount we're able to offer by putting all your items into one order which helps reduce shipping charges. Imagine trying to ship it in a bunch of packages as the items came in