There's a Kindle Rewards bonus offer going on currently.

Spend $20 on Kindle books between July 28 and Aug 1 and earn 400 bonus points. Activation required.

Those $20 you need to spend to get the bonus points gives you another 60 points, so this event nets you at least 460 points. That's enough to redeem $3 credits and you are left with 160 points out of 300 needed for another $3 credits. Just remember that points expires in 3 months and you only get 3 points for every dollar you spend on Kindle books.