Purchase flow handles cancellation, failure, and network errors gracefully

Every IAP submission is exercised by an Apple reviewer who will attempt to cancel the OS payment sheet. CWE-755 (Improper Handling of Exceptional Conditions) applies directly: an unhandled cancellation error that surfaces a crash or an error dialog is an automatic rejection under Apple guideline 3.1.1. Beyond review, a purchase flow that crashes or shows a confusing error when a user declines to buy drives uninstalls and negative reviews — user cancellation is a normal event, not an error state. A broken purchase error flow also risks leaving entitlement state inconsistent: the user may be charged by the platform but denied access by the app if the success path is never reached.

Wrap every purchase SDK call in a try/catch and explicitly distinguish user cancellation (silent, no UI) from billing failures (show a toast or alert). Verify entitlement state after a successful purchase rather than assuming the success callback equals granted access.

// React Native IAP
try {
  await requestSubscription({ sku: PRODUCT_IDS.MONTHLY });
} catch (error) {
  if (error instanceof PurchaseError && error.code === 'E_USER_CANCELLED') {
    return; // Silent — user chose not to subscribe
  }
  showErrorToast('Purchase failed. Please try again.');
}

// RevenueCat
try {
  const { customerInfo } = await Purchases.purchasePackage(pkg);
  if (customerInfo.entitlements.active['premium']) unlockPremium();
} catch (e) {
  if (!e.userCancelled) showErrorToast('Something went wrong. Please try again.');
}

Also handle network-level failures — users on flaky connections will trigger purchase timeouts that must recover gracefully without leaving the purchase screen in a broken state.