Relay is a way for merchants to route products to channels. We certainly don't want to replace Shopify (with whom we're pretty close) but just give merchants an easy way to expose product information to apps and accepts orders directly from there. In particular, Relay will not provide you with store front, or complex shipping and taxes calculations and integrations.

Ideally Relay should work seamlessly with Shopify, and Shopify users should be able to start selling on Twitter and other apps directly from Shopify through Relay without even necessarily hearing the name Relay (as it is ~the case with their payments currently processed by us)