[carpeperdiem]

James,

Let's back up a bit. When you say, "I overwrote mail/html/order_data.tpl with mail/html/order_invoice.tpl." that leads me to believe

a) you are not using an FTP client;
b) you are not using appropriate tools;
c) you are not keeping local backups;

Let's back up...

1. Run, don't walk, and do some homework on FTP clients and Text Editors for code.
2. ALWAYS work on backups. LOCAL backups, as well as on the server. You should be able to reverse any code change in an instant.
3. When working with the mail templates, oftentimes email clients will cache what was there 1 minute ago, and no matter what you send to the email client, you won't see it, if the name is the same... so clear your emailclient local cache

Finally, a tip: when posting to the forum, a subject "weirdest thing ever" is not going to attract interest... you would be better naming your post, "include not working - order_data.tpl ?"

I hope you find your solution. Get better tools ASAP and you will have a better experience w/ xcart.