If you include links in your post, they must begin with http:// or https://. All links are counted as 23 characters, no matter how long they actually are, so there is no need to use a link shortener to save characters. In fact, using a link shortener is actively discouraged.
We have some logic to truncate the toot if it gets too long. However, it ignores the fact that URLs are treated as a fixed length in Mastodon: