You'll receive an email invoice when your credit card is charged. The web order number is in the email invoice, as shown below
You can also check your purchase history [http://support.apple.com/kb/HT2727]
To view your purchased apps, click Purchases at the top of the Mac App Store.You can also view your purchase history and web order numbers on the iTunes Store and Mac App Store in iTunes.
On a computer with iTunes, click Purchase History (enter your Apple ID and password if asked). iTunes will open and show a list of content you've purchased from the iTunes Store and Mac App Store. You can also follow these steps:
1. Open iTunes.
2. Click iTunes Store.
3. If you're not signed in to the store, click Sign In and enter your Apple ID and password.
4. Choose Store > View Account. If asked, enter your password and click View Account.
5. On the Account Information window under Purchase History,click See All.
For more help, contact iTunes Store Support.
Learn how to view and manage your Mac App Store purchases.