Facebook App Consultation/Quote

I need to do a simple integration with facebook for another application. I know you can send emails to the facebook users but if they are not "your friend" it would reach the "others" folder and many people dont check it. What do you recommend/is possible to do to be able to send a notification to the user through inbox without going to the "others" folder, or writing in their wall but only for them to see it (think for example a "your order is ready, please pickup when possible" kind of message. If there is the need to generate a facebook app. What would the user need to do in order for the app to be able to post? Please describe your solution and a quote.