Personally, I would never use a debit card online, for security reasons. If your card gets hacked its your money that is stolen. At least with a credit card it is the card providers money that is stolen.
Anyway, I would delete the app and re-install. If you paid for the app in the first place you will be able to re-install free of charge. Also, the app should allow the in-app purchase to happen again as it should recognise that you have already purchased it previously.
Finally, it might be that the 3G doesnt have as much ram as more recent models of iphone and you may have applications running in the background that are preventing the app to function with the in-app purchase unless you shut down any apps eating all you memory that are running in the background. Shut-down these apps (and the simpson tap out app) by exiting to your home screen and double-tapping the home button to show what apps are running in the background. press and hold any of the open apps until it starts shaking. close all the apps down. exit then re-launch the app.