One of the selling points of Comixology's iOS app is its in-app comic book store. Or at least it was.
In a message published to its Tumblr, the company announced that it has discontinued its existing iOS app and has replaced it with a new reader-only version that lacks the older app's in-app comic-purchasing superpowers. If you want to purchase comics, you now need to go through comixology.com and then download them to the iOS app via the app's "In Cloud" tab.
You'll still be able to read comics you've purchased through in-app purchasing in this new version, but you will need to sync purchases with the new app via the Restore function. And if you want, you can continue to read your existing comics through the old app; you just won't be able to purchase new comics with it.
If this arrangement sounds familiar, it's because Amazon uses a similar purchasing setup for its Kindle e-reader app for iOS. With that app, you purchase apps from Amazon through your web browser, and your purchase gets synced to the Kindle app the next time you launch it.
Comixology hasn't said if Amazon's recent purchase of the company has anything to do with the change, but it might not be totally unreasonable to assume as much.
To make amends with its customers, Comixology also said that it is "offering a $5 eGift Card credit to all those who have purchased through the comiXology platform in the past." The company will automatically apply the credit to affected user accounts, and you'll have 30 days to use it.