Gregory's Server
You don't need to use link shorteners on Mastodon. All links count as 23 characters towards your posts's maximum size, no matter how long the link really is.
By avoiding link shorteners, you are also protecting people's privacy and reducing the risk of link rot.
(There is a bug on the official iPhone/iPad app that means links are counted as their true length instead of 23, which is yet another reason to switch to a third party app such as Ice Cubes or Mona or Ivory.)
p.s. People asking "Why 23 characters?", it's because a long time ago on Twitter links were counted as 23 characters by Twitter replacing all posted links with their own link shorteners (which meant Twitter could track what people clicked). These shorteners were always 23 characters long.
Mastodon provided a more privacy-friendly alternative to this by leaving the links intact without a shortener, but still only counted them as 23 characters.