I assume - hope even - that all songs I work on as a Mix Engineer or Composer are released as potential chart hits. Let’s be honest tho and accept that most songs will never get past some plays on YouTube (if they are smart enough to have a video) and maybe a couple of sales on Bandcamp.
It is assumed that if you are releasing a song to the public that usage rights come with it. There is no practical point at which the singer can or should turn around later and try to claim you cannot use the recording.
They can however note early in the process (not after) that they expect Credit which means that if you post that song on Spotifry or license it to a record label that takes it Top 10 that their name needs to be in the correct places for what they did, Songwriting, Lyrics, Performance etc. This should then see monies earned apportioned appropriately. If so you MUST give Credit. As credit should always be given - just as you assume that your name should always be on your song.
Example: Manfred Mann’s Earth Band releases a song called “Spirits In the Night”. It is credited broadly to Manfred Mann or Manfred Mann’s Earth Band. In the liner notes (and legal stuff) that Bruce Sprinsteen wrote the song and Chris Thompson sang the song. It will also list the other musicians and should list the Record Producer (who generally gets points) and Mix Engineer (who sometimes gets points), if not the Recording Engineer as well out of politeness as visible credit brings more work. Denying credit is denying the opportunity for more work. Mighty rude.
Many young and inexperienced people try to opt-out of this claiming that they will always own 199% of their song even tho it was made by specialists. This could get ugly later if they were not very up-front or paid reasonably - as in Scale (not $5) as they have used their power to force unfavorable terms on a supplier who could not reasonably know better.
If I feel like a client is not telling me honestly the intended use for the Work, I will move on. This is especially so if they are a business like Nike (who I am sure would always be honest seeing they know the law) yet trying to hide who they are and the real value of the work - seeing a Nike advert is worth way more than one for Shlboopy’s Shoes of Bumblyville (Population 12).
As for contracts, probably not worth a lot so long as a) you are clear what the work is and the usage will be, b) ensure that you show Credits wherever appropriate. the main Fiverr contract and your printed and agreed discussions form reasonable Intent.
If your song goes Gangnam style and the Credits are well where they should be you got your cut, they get theirs. and everyone is grinnin’.