Problem with Mercari (Yu Packet Post Mini)

I shipped an item via Yu Packet Post Mini on the 18th. I scanned the QR code but I forgot to remove that strip on the back that you’re supposed to keep. I thought it was fine because I figured that the slip is just for me to keep just in case of an issue. On Mercari it says that it was delivered on the 20th, but the customer didn’t confirm that it had arrived and just messaged me today telling me that it never arrived. Would not removing that slip really interfere with the delivery of the package if I had scanned it properly??

by ikigai9