If you handed it over the counter its highly likely they would have rejected it as not their label.

But looking at the price of postage sounds like you may have thrown it in the box out front,  in which case it may get rejected at the sorting centre,  but they cant do anything with it,  because its not their problem,

what happens if i dropped of a parcel with a sendle postage slip on it at australian postage? 

how can i get it back from australian post? 

I sold a camera to an overseas buyer who has an address in Queensland. I did not realise the buyer was in Japan and assumed it was an Australian buyer. He has paid but somehow I (or maybe the buyer)  picked Sendle as the carrier. When I saw this I cancelled Sendle as described above, went to the post office and sent  the item via Australia Post.  Sendle now say the sale is cancelled and to refund the buyer. I have notified the buyer that I sent item and have provided the tracking number but am at a loss as to what to do now. Do I just wait and hope that all ok?

Just to reply to myself, so others can find this information: I went into Activity -> Selling -> Postage Labels. There's a dropdown next to my order where you can select "Manage Delivery with Sendle". On that page one can cancel the Sendle order.