So the reason behind leaving shipster is that they want to charge you a whopping $99 a year for their own membership.
As a Kogan First member, you’ll get free delivery on 1000s of eligible products, upgrades to express shipping* at no extra cost and enjoy priority customer service.
Sorce: https://www.kogan.com/au/kogan-first/
You are entitled to free shipping on all eligible Kogan First products in selected areas (Kogan First Products). ‘Free shipping’ means free standard domestic postage (express and international postage is excluded);
If free shipping is already offered for a Kogan First Product, then you are entitled to a free upgrade to express shipping in selected areas for the Kogan First Product; and
Any additional benefits, which, in our discretion, we may offer to Kogan First members from time to time.
Sounds very similar to ebay plus
Kogan First Products are selected fixed price products available on the Kogan.com website that have been marked with the Kogan First badge. Kogan may, in its discretion, vary the range of items that are marked as Kogan First Products from time to time without notice to you.
Source: https://www.kogan.com/au/kogan-first-terms-conditions/
Kogan first, customers second.