For my testing I used:
private static final String SKU_PRO = "android.test.purchased";
From developer docs:
Product IDs are unique across an app's namespace. A product ID must start with a lowercase letter or a number and must be composed of only lowercase letters (a-z), numbers (0-9), underscores (_), and periods (.). The product ID android.test is reserved, as are all product IDs that start with android.test.
Note: Be sure to plan your product ID namespace carefully. You cannot modify an item's product ID after the item is created, and you cannot reuse a product ID within an app.