If an item is available on preorder, it means it is not officially released until that date. In terms of String Theory packages, parcels started posting internationally over last weekend to give a chance for them to get to their desired destination by Friday. (I think I will still have to wait another two weeks given the system here).
Special items with the orders (in the case of Middle Of Everywhere it was a certain pictures book) may only be available with preorders and can't be guaranteed after the release date. To be safe, order in advance by a couple of hours. My order for Middle Of Everywhere was meant to be a package, but due to a mess-up I made I was only able to order at I think 10.30PM here on the last day of preorders. By that time, the preorder option had been taken down. I could only think that maybe it was removed at the end of the work day in America.
Certain special items will not be made available later. I doubt whether the lyric book for String Theory will be for example, and most certainly something like the Playbill won't. I also doubt very much whether A Piece Of String (the instrumental version) will be available afterwards. It all depends on what you want.
In terms of the DVD "The Theory Of Everything", it might be available separately, but that said, a DVD like "Watch From A Distance" was only available with a package, and I presume it will be an item like that.