Client requested an Amazon Webstore

Annnnnnd it's deprecated. Question: what is the alternative? I don't even know what an 'amazon webstore' is, is it just Amazon's old competitor to Shopify (I see they're recommending Shopify).

My client already has a Wordpress website so I don't see any reason to migrate everything over to Shopify. I'm just trying to understand the point... is it to use Amazon as the CIM/Payment Processor? In that case, does Amazon have an API and service to manage that? If so, what should I be looking for? Thanks.