This is not correct 

I tried many time to cancel a label even 1 h after I created it and simply there is no option 

- only have 3 options in the action bar but no cancel option. The 2 postages numbers I did not used where replaced by 2 new ones still with the Post and I still have them in my archives. Cannot keep going like that for a small business, postage $ 9.15 charged twice for a simple item at less than $10.

This is not the first time I have issue with the Post without  even thinking the time my client could not collect her parcel (emailed twice the Post service and simply my package disappear somewhere  nobody at the Post can find and worst, no one replied to my emails)

Maybe it is not possible any more, but curraone posted in 2014 (apparently copying and pasting from an eBay page).

I am having the same problem. I clicked on Print Label to find the buyer's address only to discover I had bought a label. The cost of the label was $8.83 which is a lot more thant the $2.20 it costs to post a CD as a letter. If I remember correctly the label page said that I wouldn't be charged until the label was used, which is not going to happen. I can find no option to cancel or void a label as described in other posts.