Good point, but I want to clarify/qualify your point a bit further:
First of all, Shipping is based on Zip code, so it may be better for some to go with Amazon and worse for others
Second, these guys have tons of other batteries so it is quite concienvable to have a $100+ order, which makes shipping free...assuming you have a laptop, etc as well.
I should also mention that if you are going to buy 5 or 10 of these, you may want to call to see if you can get a bulk discount (sometimes 3 to 5 percent or higher depending on quantity).
I don't disagree that Amazon is probably a better bet for a Sharp-branded (as opposed to compatible) for the BL06 battery, but with all this site has to offer (and since Sharp is no longer selling in US) you may want to consider buying a few if you think you are going to own the PDA for a few years. But that just my opinion.
Note: They aren't always the cheapest on everything, so make sure to check ebay and other sites (like amazon) for any other batteries you may decide to buy.