Was it during business hours when you tried contacting eBay? (and did you try by using eBay Help or some other way?)
Have you tried again (you may be very lucky and someone from eBay staff may read your post)
Was it the correct QR code (not mixed up with another seller for another item)
Was it an issue with the QR code itself, or the app, or the phone?
Which 6 digit figure?
Is there not an option to 'mark as sent' ? (since you also offer 'free' postage on items?
At the moment I am more confused as to how you are posting books untracked regular letters
