It can work. But you have to work around the games Yahoo and WordPress play.
To add an order button to a webpage, Yahoo's Storetag wizard tells you to use text like this
'<form method="post" action="<!--#ystore_order id="youritemid" -->"><input type="submit" value="Order"></form>'
The problem is here
Yahoo's servers dynamically transform this when the page is actually served to a web viewer. You can tell this happens by actually going to any page where you already have placed a Store Tag and "View Source." Note how the Store Tag now looks something like this:
<form method="post" action="http://order.store.yahoo.net/cgi-bin/wg-order?yourstoreid+youritemid" onsubmit="return checkData();"> <input type="submit" value="Order" /></form>
THIS is actually what you want to insert into your WordPress blog. By doing so, you will get an Order Button that actually posts an item straight into the shopping cart.
Now, I've not tried to troubleshoot things like items with options so who knows how that might work. But this should at least get you started until someone can solve this as a WordPress plugin.