How to add a mac OS service item directly in the contextual menu instead of services submenu?

It has nothing to do with your service in particular. Services are displayed directly in the Finder context menu if there are four or fewer relevant services available, and in a submenu if there are five or more:

Note the extended “Services” submenu has also been merged to become part of the general right-click menu because it dropped under 5 items. That’s because the “Services” section becomes it’s own dedicated submenu once more than four items or Service options are available for a given file, filetype, or folder, but with under five items, the Services menu blends into the general right-click menu of OS X.

-- http://osxdaily.com/2013/05/14/remove-services-contextual-menu-mac-os-x/