How to reproduce:
1. Create a new Unity project
2. Open the https://assetstore.unity.com/packages/vfx/shaders/free-skybox-extended-shader-107400 in a browser
3. Select "Add to my Assets" and "Open in Unity"
4. Observe the error in the Package Manager window
5. Click the "Download" button in the Package Manager window

Expected result: No errors appear in the Package Manager window, Assets are downloaded
Actual result: Error "An error occurred: Unable to get purchase details because you may not have purchased this package." appears in the Package Manager window

Reproduces on: 2019.4.9f1, 2020.1.4f1, 2020.2.0b2
Cannot test with: 2018.4.27f1 (Asset store is in the Editor)

Note:
- In the Unity 2019.4.9f1 the error changes to "The requested URL returned error: 404 Not Found"
- Affects non-internal accounts with paid/free assets, users unable to download or install