Law / ZUS for a sole trader in Poland [10]

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ZUS for enterpreneurs who are also employed?

I'm planning to star a business and I'm also employed with "umowa o zlecenia" contract.
I've ready everything relevant on "biznes.gov.pl" site but couldn't find much information. Do I need to pay any ZUS (as it's already paid by my employer) or is there some sort of flat fee for it?