Getting payment invoices
When you make a purchase on Google Play, you should get a confirmation email with your order information to the Google Account you used when making your purchase. You can learn more about it in this article.
If you want an invoice/receipt of payment for your Contacts+ Pro subscription, you can download/view it from your Contacts+ Pro WebApp.
Sign-in to your Contacts+ Web App (Click here) > Click on the 'My address book' section (on the top right corner of the page) > Select 'Settings' > Go to 'Subscriptions', and scroll down to view/download the receipts.