How to decide which AWS region is the best to launch an e-comm website?

Hello.

I think it's a good idea to choose a region based on cost, communication latency, and feature support of AWS services.
Basically, if you are targeting within the United States, I think that if you select one of the four regions listed above, you will not have to worry about such a large communication delay.
Next is the cost, which may vary slightly depending on the region.
For example, the document below lists EC2 prices, but the "us-west-1" region tends to be slightly more expensive than other regions.
https://aws.amazon.com/ec2/pricing/on-demand/?nc1=h_ls

Next, regarding the features supported by AWS services, the latest AWS services and features tend to be released early in "us-east-1".
So, in your case, the target is users in the United States, so I think there will be no problem for the time being if you select "us-east-1".