I have a Twitter account where I have a contractor posting tweets, and I don't want to give the actual Twitter password. I can set up Hootsuite or Buffer so the contractor can post tweets without knowing the Twitter password because they instead know just the Hootsuite or Buffer details, and those accounts I've previously connected to Twitter. But those services use non-t.co shorteners (ow.ly and buff.ly, respectively). Hootsuite does support t.co if you include the full URL, but then it gets the character count wrong (it doesn't take into account that the link will be shortened).
So my question is, are there any other third-party tweeting services which (a) have their own login mechanism, so the twitter password doesn't have to be given out; (b) support t.co links; (c) get the character count right, i.e. take into account that longer links will be shortened by twitter.