How can I get an invoice after my purchase?

All purchases are processed through the app store, so invoices and receipts come directly from them:

For iPhone (Apple):

Apple sends purchase receipts to the email address associated with your Apple ID. You can also view your purchase history at reportaproblem.apple.com.

For Android (Google Play):

Google sends purchase receipts to your Gmail. You can also view your order history at play.google.com/store/account.