Sandbox mode not enabled for Steam microtransactions

Hello all. I'm making progress with microtransactions in Steamworks.NET, but I have a new problem. The overlay shows when calling SteamInventory.StartPurchase(), but I don't appear to be in sandbox mode. My Steam Wallet is empty, so I only see an "Add Funds" button when the purchase window opens. The window displays the name of my developer account, which is also returned when calling SteamFriends.GetPersonaName(). Development Build was enabled in the Build Settings.

The ISteamInventory Interface page makes it sound as though all I need is to be logged into my developer account, at least until the app is released. Does anyone know how I can test in sandbox mode for a game that's already published?

 

I contacted Steam directly, and they confirmed there is no sandbox testing for released apps. The only thing they could suggest is requesting refunds for items not consumed by the game.

 

Can you share your experience on this topic.
This link has an example on this topic using Facepunch, but again the whole process is not explained there.
https://wiki.facepunch.com/steamworks/Item_Store_Cart
And I couldn't find any examples on the topic of microtransactions with Steamworks.NET
https://steamworks.github.io/
Thank you in advance )