You can use almost everything that is in the Asset Store commercially. However, there may indeed be some Assets that are not allowed to be used commercially - such as Unity’s Adam assets.
While I personally wouldn’t use the Standard Assets anymore (since they are outdated), you can freely use and modify them for your own needs.
Basically, what you can’t use commercially are those Adam assets I was talking about or anything from The Heretic. These are not licensed under the Asset Store EULA.