Hi @TenacitatemSors,
This appears to be an intermittent problem that affects some players, and seemingly ‘fixes’ itself a short while later. Are you still unable to see or purchase the Sister of the Thorn Cosmetics Upgrade now?
It’s a rather strange issue, but we’re aware and looking into it!
In some regions, prices can drop below Steam’s pricing threshold which can also result in a DLC being unavailable to purchase, though I’m unsure if this is the case here