If your song is manufactured and sold by a third-party (e.g. someone covered your song), you are entitled to a mechanical royalty. This will be paid to you by the third-party releasing the album for sale – normally a record label or artist.
Then, as uses occur, mechanical collection societies, such as the Harry Fox Agency, then collect income and distribute it to Songtrust as your Publishing Administrator.
Songtrust uses the Harry Fox Agency in the US to obtain mechanical licenses for our clients. We do not provide any services related to direct payment of mechanicals from a third-party to our clients.