SKPayment *payment = [SKPayment paymentWithProduct:electronicProd];
    [[SKPaymentQueue defaultQueue] addTransactionObserver:self];
    [[SKPaymentQueue defaultQueue] addPayment:payment];

- (void)viewDidLoad {
   //... stuff
   SKProduct* electronicProduct = //...
   [electronicProduct retain];
   //... otherstuff
}

[[SKPaymentQueue defaultQueue] removeTransactionObserver:self];

- (void)viewDidDisappear:(BOOL)animated
{
    [super viewDidDisappear:animated];
    [[SKPaymentQueue defaultQueue] removeTransactionObserver:self];
}