This is a problem that we get reported from time to time, even from people who have typically just used one Apple ID in the past. The issue that you're seeing is a common one that relates to the Apple ID that was used to install the app on your phone. The problem actually isn't with Hipstamatic at all, but it has this effect on your attempted purchase through the app.
The problem, in nearly every case, is that you're not signed into your phone with the same Apple ID that you used to buy Hipstamatic (and your paks) originally. Because of the way Apple's billing system is set up, you must use the same ID that you used to buy the app to make any additional in-app purchases or to restore past purchases. This is Apple's policy, not one created by Hipstamatic.
You can easily check which ID your phone thinks you want to use at any time, by just going to the App store app, tap the Featured tab at the bottom left corner and then scroll all the way down to the bottom of the screen. It'll show you which account is signed in.
(Even if you've stuck with one Apple ID over the years, what many people find is that they've accidentally ended up signed in with an iCloud or me.com account, rather than their normal Apple ID that they use for purchases.)
So check the ID you're currently signed in with. If it's not the one used to buy Hipstamatic originally Just take a moment to sign out of the that Apple ID and sign back in with your original email/ID. Then open up Hipstamatic and try to make your purchase or restore again.
As noted, that's typically the cause, and so you should be able to buy paks at that point. Once the download is complete, you can switch IDs again in the same way as described above and go back to using the newer ID.
If that information doesn't help, and you're still getting that message, please send us an email to: firstname.lastname@example.org including your Hipstaccount email address, your iTunes email address and any background information you can on what's happening at your end. We can sort out those issues for you and make sure your gear gets restored and/or your purchase is successful.