Hi there @LingLing.
Welcome to the Community.
Promotions such as these can take 1-2 months to come into effect after your monthly charge date.
In regards to this promotion specifically, it doesn't work as a $50/month plan with a $30 credit resulting in a $20/month charge as you expected. You're receiving a $30/month additional bonus balance to your top-up each month if you've met the eligibility requirements.
A good way to find out if you've received the bonus is to go on My Account under your payment history to see the amount that you've topped-up and the amount of time your plan payed itself and compare it to your current available balance.
Keep in mind as well that if you do miss a payment, the promotion will fall off.
Let us know if you have any further questions.
