You cancel the subscription in the settings app of your iPhone and tap on your name at the top. Then go to "Subscriptions" and locate Smart Receipts to cancel.
Apple, unfortunately, does not allow developers to issue refunds directly through their system. You can try to contact their team through here to see if that helps: https://support.apple.com/en-us/ht204084.
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.