$10 for 10GB is advertised on Amaysim's own site and lasts until the 31st of January.
$30 cashback is from ShopBack and only for 3 days.
- Cashback is only valid on new services and for new sign ups/customers (SIM)
- If however the service (SIM) is not activated within 30 days of purchase, the sale will be deemed cancelled and cashback will not be awarded.
Bear in mind Amaysim ships their SIMs activated, so even though you paid for 28 days, you'll likely get closer to 25 days of real use out of it (even less if you order late in the working week and have to wait over the weekend).
Auto-renew is on by default, make sure to turn it off at https://accounts.amaysim.com.au/identity/login.