Twitter only allows for messages of 140 characters. Tweets being shortened is a sign that your Tweet is over 140 characters.
We truncate at the nearest word boundary which will allow the message to be posted. Word boundaries are punctuation, spaces, or numbers embedded in words.
Any prefix, suffix, links and/or auto-hashtags that you set up in dlvr.it count toward the 140 character limit.
Twitter adds their own short URL (t.co/xxxxx) to each link. Twitter URLs require exactly 23 characters. It doesn't matter how short or long the original URL is - all URLs posted to Twitter count as if they are 23 characters.
This also includes domains that don't have the "http://" on the front, so "YourDomain.com" counts as a URL.