This has been around of a while, but since I manually update, and I rarely shop online, this gets updated rarely.
??? -> 5.2
Version history, items of interest.
Multi-device shopping in a single session (even PC, which I assume means web)
"one-touch checkout" with Apple Pay (which I assume is via TouchID)
support of countries.
support for Apple Watch
Apple Store Gift Card
iPhone Upgrade Program
app will recommend new products based on your current fleet.
iOS 10 "rich notifications"
fulfills "Trade-in Kit"
(Apple Pay note above might be wrong...) "You can use TouchID to securely..."
credit card scanning.
"Favorites" in stock
Invoice management, inc. cancel *an* item.
use Siri Shortcuts to control upgrade program.
Siri Shortcuts to check product availability.