There are multiple definitions - click on the translation that matches your context.
Zuschlag means:
Substantiv
full
noun
This English term may be used in an employment law context when an extra payment or benefit is bestowed on an employee; e.g., for a "Kinderzuschlag" (child supplement) or "Schwerbehindertenzuschlag" (supplement for severely disabled individuals)
a sum of money that you have to pay for an extra service or thing
There's a supplement of $50 for each additional person.
something that is added to a book, report or other document etc to complete it or to improve it
Did you get the supplement to the guidelines I sent this morning?
something that is added to something else to complete it or to improve it
The case has opened the debate about drug testing in sport and the use of supplements.