My name is Dallas and I work for the Social Care Team here at Shopify.
The 'Buy Now' button, aka Dynamic Checkout, was created for customers to have a quick option when they are just purchasing one item. If you were wanting to have them check out with multiple products then you would just need them to use the actual checkout button.
This document here goes into more detail about the Dynamic Checkout and how it works.
I hope this answers your question. If you have any additional questions please let me know by replying to this forum post.