Four years ago my starting credit limits for Walmart (store card) and Amazon store card were the exact same as yours $400/$800.
Fast forward and as of last month my Amazon was $10,000 and my Walmart store card is now a Walmart MasterCard at $25,000. The Amazon should be $15K-$20K by August 31st or September 31st when I ask for my next CLI with them.
Patience is a b*tch, but she puts out big time in the long run. 