Go ahead and buy the box product from whomever you want and use the 25 character PIN from the box to renew your subscription. I've done this myself and it's the only way to get around using a credit card if you don't wish to use one.
The reason for the price difference is that the online renewal is provided at list price, while the box product is often discounted by the various retail stores in an attempt to compete with each other. If you think for a second it's quite obvious that if Microsoft sold it online for the same or less than in the stores, almost no one would ever purchase there. In that case, how long do you think it would be before the stores stopped carrying the product?
Channel sales are a common concept in the sales distribution world and though some companies have been known to bypass their resellers and cut them off from sales, they almost always end up destroying their market share in the long run, since no one will trust them again. This is a simple and common concept in the maketing world. Microsoft does provide volume discounts for large corporate customers, but since home/SOHO sales tend to be individual it's really not effective to compete with their own reseller channel for this market, since they need the resellers to introduce the products to new customers in the future.
OneCareBear
