First, I'll refer you to the WordPress section of this help page, although it may only allow you to understand what is happening without really helping you avoid the problem:
http://www.e-junkie.com/ej/help.site-blog.htm#wordpress
The problem is that the visual-design mode of WordPress' standard posting editor tends to mangle the javascript portion of our standard button code. When you paste-in our code and save the post directly from the code view, that should preserve the code exactly as you pasted it, but then if you edit the same post later, our button code gets mangled again as soon as the post is loaded up in the post editor's visual-design mode.
The simplest way around this would use our non-javascript button codes, although this would open the cart in a new window/tab rather than using our nice overlay-style standard cart. Aside from that, perhaps you could experiment with using a different posting editor plugin that may be able to handle existing button codes without mangling them?
