I think the FedEx/Walmart merger is more likely by the day. There will come a day very soon where shipping becomes a commodity or even loss leader. It'll be all but impossible to survive as a standalone shipping company. UPS might be able to survive on its own just because of its deep roots in supply chains and traditional businesses but it'll be a changed company in 10 years.
Walmart needs a high speed air freight network, FedEx needs a way to not have to make money on e-commerce shipping, and Shopify supplies the 3rd party seller network.
The big variable is if the resulting company can replicate whatever magic Amazon has developed to make it all work as well as it does. They have the benefit of starting over from scratch with the logistics network and have tremendous technological advantages right out of the gate. Walmart has a ton of money to throw at this. FedEx will find themselves increasingly motivated to change culturally to meet the new reality. Shopify will have to deal with no longer being a neutral player that small business can trust to not become a competitor like Amazon does to their 3rd party sellers that get too big.
It could work great, or it could be a boondoggle of a corporate merger for the business school text books. But I think ultimately it's something these companies are going to quickly find themselves forced into whether they like it or not.