If you are operating as a reader app then consider the following comment:
In bottom part of that page there is a "Cart" icon, which will add model into cart.
Is the purchase of the contents of the "cart" done through IAP or by other means? If it is done by other means then it may be in violation of this requirement for a reader app:
you agree not to directly or indirectly target iOS users to use a purchasing method other than in-app purchase
As I understand your process, after a purchase the user just gets an email with a link. The contents of that email provides access to a model. The email is read outside the app, the model is viewed outside the app and the only difference in the app is that the purchase or download button changes. If that is correct then I would recommend you abandon 'reader app' and appeal the rejection stating:
"The app displays a complex catalog of models that can be or have been purchased by the user. Once purchased, a model is not used within the app nor is the display of the model within the catalog changed in any way. Once purchased no code is unlocked nor is any functionality changed within the app. The app does generate an email that provides access to the model. But that access is not through the app. The user downloads the model using their mail program and can transfer the model to a file in their computer. The model is never transfered into the app, read by the app nor used by the app. Therefore, the purchase does not 'unlock features or functionality within (the) app' and Guideline 3.1.1 does not require the use of IAP. In fact, guideline 3.1.5(a) may explicitly preclude the use of IAP were these models considered to be a 'Good or Service' because they are, in fact, 'consumed outside of the app' as referenced in 3.1.5(a)."
Here's what could happen
1) App Review responds that the app is used to distribute the access key - that email with a link in it - of the model and that access is unlocked through teh purchase and must be done through IAP
2) App Review ignores #1 and approves the app since most of the value is in the model not the distribution system.
3) App Review does not understand and rejects the app
Or use IAP. And point out that what you are selling are 'digital goods' not 'Goods and Services' as defined by 3.1.5(a).
It would be ironic if you added IAP and then got rejected because of 3.1.5(a).