You cancel the subscription by going to the Google Play Store on your Android and tap your picture in the top right. Then tap on "Payments & Subscriptions" and locate Smart Receipts to cancel. https://support.google.com/googleplay/answer/7018481
To delete your Smart Receipts account, you will need to tap the 3 lines at the top left of the Smart Receipts app. Go to Settings-->Support-->Delete Account Request.