That's because Shopify is a tool for sellers. Buyers are mostly unaware as to whether a seller is using Shopify same as you probably don't know if a maker is using 

Stripe

or 

Wordpress

 on their site.

 having a seat on Stripe's cap table is a privilege reserved for those who are called privileged by those who are privileged. 

Dope line right there.

I think all of this makes sense to me. Both of them think they can take a good stab at that functionality... why not go for it? In the end they know that they're aligned whoever wins. 

Good job with connecting 

Stripe

and 

Shopify

in this 

skin in the game

manner. To me I think of Stripe and Shopify as fighting to arm the rebels -- 

Star Wars

reference -- against the empire -- aka likes of 

Amazon

  / 

Walmart

 and whatever other juggernauts await us.