To cancel an unpaid invoice and issue a credit note:
1. Open the Countingup app and tap 'Accounting' at the bottom of the screen
2. Tap 'Sales invoices'
3. Tap the 'unpaid' row
4. Swipe left on the unpaid invoice you want to cancel
5. Tap 'Cancel'
6. Confirm by tapping 'Cancel invoice'
7. Send a credit note to your customer and choose if you want to send a copy to yourself with the check box
8. Tap 'Send' to issue your credit note
