Twitter only allows for messages of 280 characters. Tweets being shortened is a sign that your Tweet is over 280 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 280 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.
You may also be experiencing an issue with the 280 character update. You can review a lot more detail on our blog.