Contest for iPhone app purchasers -> How can they prove they bought the app?

I'm thinking about holding some kind of contest/giveaway for people that bought my app, basically as a marketing strategy. Buy the app and get a chance to win $ or some prize. Not sure of the legalities on all that but I'll check into it.

Anyway I don't have any feature in the app to support this or anything (and its already 开发者_运维技巧launched), so I was hoping to just have people submit their order # and email address or something. But from the looks of it, although buyers do get an Order # back from Apple, the seller doesn't get ANY info about who bought the app or what their order #s are, so there is no way to x-reference anything. Please correct me if that is incorrect.

This is in contrast to the Android market where both buyer and seller get the order # (buyer via email and seller via Google checkout purchases screen).

Let me know if you have any thoughts or ideas that could help. Ultimately I guess I could release an update to give the users a screen in the app to support this, but was hoping for something I could use without that.

EDIT

A bit of further research indicates that my requirement of people to buy the app to be in the contest would probably cause the contest to be considered an illegal lottery. Oh well :<

There isn't really a surefire way besides doing your latter idea and implementing it inside the app.

Implement a unique promocode in the app. Use the ecid + some magic to generate a short code. The only problem is that you have to update the app.

This may or may not be legal, depending on how you implement it. IANAL, but contest are considered gambling when they included all three of these elements:

• Prize
• Consideration (require some sort of payment to be involved)
• Chance

Remove one of the three and you should be good. Note how many contests, for example the silly Monopoly game run through McDonalds, always state "No purchase required" and provide for a way to play for free. This removes the "consideration" element from the mix.

Edit: I am making the assumption you are in the US. In any case, this is probably a great question for a lawyer.