Once you have purchased an asset from the store, it becomes yours to do with as you like within your games and apps. You will be able to use them in your game for commercial use with no extra payments.
You can also use free assets in your game for commercial use. Using selected (non-restricted) Unity assets is fine, though be aware that many developers use Unity to create their games and most are aware of the assets we provide.
Unity "Restricted Assets" are subject to restrictions that others are not. These are only valid for personal and non-commercial use only. No other use is permitted, and these Unity assets must not be used in your commercial games.
You cannot duplicate an asset, or include it in an asset-type product.