You can't make calls to the Shopify api without an access token, this will be the token that you saved when you first authorized the app. If you aren't using the API at all, then I assume it won't be possible to uninstall via the API (without first generating an access token, which your client would have to authorize). Not 100% sure though...
The Shop Front: Welcome to the eCommerce platform with integrated video, audio and chat. Engage with your customers online and offline with multi-channel conversion tools. Edit your products and prices in realtime and stream them privately. Available in the App store: https://apps.shopify.com/the-shop-front
