You'd have to contact account/billing support at https://support.roku.com/contactus for a refund, but before you do make sure you restart your Roku device from Settings->System or Settings->System->Power (as appropriate) and give it another try.
The other thing to try is to remove the app by highlighting it on the Home Screen, pressing * on your remote, and choosing "Remove app". Restart your Roku as given above (don't skip this), and then add it again. I hesitate to suggest this because I don't think you'd be charged again, but I'm not positive.
By the way, I can't find anything called "Virtual Snowfall" in the US Channel Store.
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