Order Summary Solution with Flexbox and CSS Grid

Hey, both are good, they have slightly different use cases which you will see yourself as you practice more, believe me :) Here's an article though, at the second half it compares flex and grid: (https://blog.logrocket.com/flexbox-vs-css-grid/). You may find this useful :)

Your solution is fine though, as it has been said by @Teegamtee, you could give it a smaller max-width. Also, a background-size: contain would fix the background wave :)

It doesn't matter what you use as long as it works and does the job.

& by the way your solution looks ok and I think it would have been better to not have stretched it when it reached 990px.