From what I understand, Sendle is more like a broker - they send a courier to pick up, but the courier could be from anywhere. Their customer support is awful.

My husbands business has been waiting for a scanner for over 5 weeks. I kid you not when I say that the item has been palmed from one courier "partner" to another until at least 4 different companies have handled this item and now it is lost.........

I've been using Aus Post exclusively for my items and have had very few issues. My buyers are paying for the postage, so I don't find it worthwhile to try and find a cheaper courier. At least with Aus Post I will always get a response from CS and a resolution to any of the very few issues that have cropped up