What you should have done was cancel as unpaid after 4 days, that has no effect on your account and the buyer can't leave feedback.
You now have a problem caused by the buyer's late payment as you won't be able to use the simple delivery label paid for by the buyer as it will expire before you get back. That means buying another one from the sales proceeds as the buyer is the one that gets the refund for the unused label.
I think you should tell the buyer that the late payment has caused difficulties with the postage and that you would like their agreement to cancel the sale. You need their agreement as a seller initiated cancellation would give you a defect and leave you open to negative feedback.