Am not have trouble buying on my app

Are or you aren't you having problems ? If you are then what problem are you having e.g. what happens when you try to do the IAP, does the 'buy' button not work, do you get any error messages ... ?

If you are getting a message to contact iTunes Support then you can do so via this link and ask them why the message is appearing (we are fellow users here on these forums, we won't know why) : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption

If it's a different problem ... ?