Interesting, yeah could be. Disabling Play Store auto-updates for the app if enabled might help.
Actually I'm not sure how you installed Amazon Shopping from the Amazon store anyways? I installed it a while back assuming they would have versions of Shopping and Kindle that would let me buy books, but they didn't seem to distribute those apps at all which I thought was strange. Still not seeing them there now.
Wow, you're right. Now I'm not sure of my own memory on this. I remember installing Kindle (or pre-Kindle Comixology...?) from somewhere and being able to buy on it, but then it got replaced with the version where you couldn't. Maybe that was because it got discontinued on the third-party app store (which I thought was Amazon's)?
Actually I'm not sure how you installed Amazon Shopping from the Amazon store anyways? I installed it a while back assuming they would have versions of Shopping and Kindle that would let me buy books, but they didn't seem to distribute those apps at all which I thought was strange. Still not seeing them there now.