Venus -- this is what the website says about the preorder package:
Tales of Monkey Island will debut July 7 on PC, and we've put together a sweet pre-order deal to entice you to buy the full five-episode series up front. How sweet, you ask? Well, in addition to getting five months' worth of Monkey Island adventure, if you pre-order before July 7, you'll also receive:
A FREE Collector's DVD at the end of the season, with a special, limited edition cover painted by legendary Monkey Island artist Steve Purcell.*
So it sounds as though there will be a DVD version, with a limited edition cover for those who preorder (probably a different cover for those who don't preorder). In the past, in order to get the DVD at the end of the season, you had to play the price of shipping -- I expect that's the same here as well.
So it looks to me as though you could wait, if you choose, and buy the DVD version later when the episodes have all been released, though I don't know if the price will be different by then or not.
EDIT: I just found more info in the fine print.
*Season customers who order after July 7 will still be eligible for the DVD, but only pre-order customers will receive the limited edition cover. Shipping will be charged when you come back to order the Collector's DVD.