Is there any way I can get my money back then, seeing as there are no built-in stealth mechanics for the OpSive controller and I'm going to need some sort of stealth dimension to my game
You can create your own ability for that use case. From the character controller side of things it's easy as it would just be a different animation. It would then be up to your particular project to determine when the character is determined to be stealthy versus not.
In terms of refunds we follow section 2.9 of Unity's EULA: https://unity.com/legal/as-terms. If you haven't downloaded the asset please send me your invoice number to support@opsive.com and I can send in the request.