You find your invoice in your account when you sign in through your browser.

1. Sign in to www.roomsketcher.com.
2. Click Account in the upper menu.
3. Click View Payments on the left side.
4. Click on the blue text of an item to download the invoice. 

If you have made your payment via in-app purchases (Apple or Google Play Store), you receive the invoice via email either from Apple or Google.