I just had a random look at a few of your items for sale.

Postage says:  Australia Post standard parcel.

If I were to purchase from someone who stated that and then sent thru Sendle I would give an immediate NEG.

They are not a Courier but a broker and pass on to the cheapest courier they can find. 

If you look through forums you will see numerous threads complaining about them. 

If you choose to use courier service, find one that you trust and use them but make sure that it is noted in your listings.