You have 2 options:
1) ask that plugin support what permission is required for access to its menu;
2) share plugin copy with me (support [at-sign] role-editor.com) via DropBox or similar service in order I can find an answer myself.
All items of ‘WC Invoice’ menu are protected by ‘manage_options’ user capability. It’s hardcoded in plugin source code. Plugin does not provide any filter to change this capability without editing source code.
Admin menu access add-on allows to block for a role critical menu items, which you can not block via user capabilities.