Hello, im selling my musics on fiverr and i compose, produce and sing it from A to Z. So about the copyright, i know that the buyer has full copyright on the product when i choose commerciale use. But if i want to specifiy a certain percentage of royalties i want to get if the person wants to sell the music, should i write the percentage and the roaylties term on the gig info, or should i do a real contract with royalties info that the person will sign?
When they purchase commercial rights to the copyrighted work that is to give them full copyright. You are not entitled to royalties in that case, since that is the purpose of them purchasing commercial rights.
You have no way to enter into a binding contract with your buyers on fiverr since you cannot trade contact information with them.
Your full payment is obtained when they purchase your gig.
Aside from that no one will be buying music from you that includes them having to pay royalties.
Anyone who is good enough to get royalties from their music should have their own agent and lawyer and be selling music successfully someplace already.