Open ‘Shop Manager’ role at Users->User Role Editor.
Revoke ‘activate_plugins’, add ‘manage_options’ capability. Click ‘Update’ to save your changes.
Test your changes. You will see that user got access to the ‘WitFTP’ menu item. But he got access to the ‘Settings’ and possibly other menus also.
That’s why I recommended you to activate ‘Admin menu access’ add-on and block with it menu items unneeded for the ‘Shop Manager’ role.
Let me know the result and if you need further help, including live site.