I have some background in retail loss prevention. Here's my advice.
1) Document five or six of the cancellation incidents. Depending on what you can pull, you definitely need dates and times. You need amounts. (I'm assuming these orders are being canceled for cash.) Transaction numbers will be a definite plus.
2) If your company has an anonymous tip line, use that. Every retail company I worked had one. If your company doesn't have an anonymous tip line, write up the details you have and mail it anonymously to your corporate office, ATTN: Loss Prevention.
3) Stop digging around in other stores' computers.
I disagree vehemently with Christopher that coming clean completely will benefit you. LP will take a dim view of you taking advantage of security holes to look at the software and in other people's computers, because they'll want to know everything that you did and they've unlikely to believe that all you did is look.
Your best play there is to wait about three months and then send the corporate office another anonymous letter, this one explaining the security holes and how they can be taken advantage of. You may even want to go so far as to have it mailed from another city (and that's easily accomplished on the 'net -- you write the letter, and have someone in another city mail it for you) so it can't be connected back to your location or the earlier anon letter that outed the thief in the other store.
That's my advice. Out the person anonymously.