Limited 2 per customer.
I am not sure how many months it can last. Will previous method such as change the plan to $21 pm or what so ever can still be used?
Free 2hr delivey as well.
By the way, this can also be used to "charge you existing account" (which i believe is a grey area and only available once per 6 months) by "activating replacement sim" option.
To make it last 3 months you need to pay an extra $7 now. So $45 + $20 referral equals $65 credit.
Still $27 for 3 months isn't bad if you want a break from Sim slutting monthly but not much data (39GB). The good thing is data slows to 1Mbps if you use it all.