Instead of linking to your storefront, you can link customers to a specific plan that is available from your storefront.
to the end of your URL, then paste your plan ID after the = sign. Your final URL should look something like this:
Now, whenever a user follows the link, the plan will be automatically selected and Flexcheck will move the user on to the next stage of the checkout process.
If you have the "Allow Cart Checkout" option enabled in your Flexcheck settings, the plan will be added to the user's cart, but Flexcheck will stay on the storefront page to allow them to select additional products.