Hi,
I am just doing this now myself, so far so good, what I did find is that STS and Extra fields do change some common files, so you need to be EXTRA careful to show this info. Confirm that you have done it carefully, and made sure that after you installed STS that you manually updated the files for Extra Fields.
First question, have you included the $pef$ tag in your product_info template?
Will help some more once I got mine up and going, as I have a "Table does not exist" problem........
See you round