First disclaimer: I'm not a lawyer and this isn't legal advice. You should rely on your own judgment or that of a lawyer's in deciding whether to use your affiliate tags on StoryOrigin.
StoryOrigin does not and cannot guarantee that it is in compliance with Amazon's TOS as they are regularly updated. The best I can offer is that StoryOrigin has been designed to comply with Amazon's affiliate program TOS.
Third disclaimer: When I refer to Amazon's terms, I am talking about Amazon.com, but Amazon affiliate program terms vary by region, so you should make sure the same terms apply to the affiliate program you're a part of.
Before we get dig into Amazon's policies, it's important to mention how StoryOrigin's Universal Book Links ("UBLs") work from a reader's perspective.
Step 1) The reader clicks a UBL and is shown a page listing the various retailers where the book is available.
Step 2) The reader then clicks on the retailer of their choice.
Step 3) The reader may choose to be redirected straight to the retailer or to set that retailer as their "Preferred Store." If the reader sets that retailer as their "Preferred Store," they will automatically be re-directed to that store on subsequent visits to UBLs where that retailer is an option.
Okay, with all that said, let's jump into the specific sections where people typically have concerns.
For that, let's look at section (v) and (w) of the Policies page (emphasis added).
(v) You will not cloak, hide, spoof, or otherwise obscure the URL of your Site containing Special Links (including by use of Redirecting Links) or the user agent of the application in which Program Content is displayed or used such that we cannot reasonably determine the site or application from which a customer clicks through such Special Link to an Amazon Site.
(w) You will not use a link shortening service, button, hyperlink or other ad placement in a manner that makes it unclear that you are linking to an Amazon Site.
I think most people probably read the first few words of sections (v) and (w) and immediately write-off link shorteners / redirect links, but take a look at the bolded words.
Here, Amazon essentially saying - it's fine to use link shortners just as long as we can tell what site that reader came from.
StoryOrigin doesn't do anything to obscure that the reader came through StoryOrigin, so we should be good there, but what about section (v)?
When a reader clicks on a StoryOrigin Universal Book Link, they will see the various retailers where it's available. When they click on the "Amazon" button the reader will be taken to Amazon. That's as explicit as you can get!
I think a lot of people assume that universal book links wouldn't be allowed, because section 4 ("Promotional Limitations") of the Policies page (emphasis added).
Promotional Limitations You will not engage in any promotional, marketing, or other advertising activities on behalf of us or our affiliates, or in connection with an Amazon Site or the Associates Program, that are not expressly permitted under the Agreement. You will not engage in any promotional, marketing, or other advertising activities in any offline manner, including by using any of our or our affiliates’ trademarks or logos (including any Amazon Mark), any Program Content, or any Special Link in connection with email, offline promotion or in any offline manner (e.g., in any printed material, ebook, mailing, or attachment to email, or other document, or any oral solicitation).
However, remember the process from a reader's perspective when clicking on a universal book link.
When the reader clicks on a universal book link, they are taken to a page listing the various retailers where the book is available, so they are now "online" and they are viewing the book at various retailers, not going straight to Amazon.
From that page, they click on a link that resolves to the retailer's website. Thus, the affiliate link is never included in the email itself.
It's only after the a reader has set their preferred store that they would ever get redirected straight to Amazon, but that would be up to the reader's choice to do that.