With variable products, the syntax to send any particular variation directly to your custom checkout is slightly different than simple or subscription or grouped products.
You need to add &variation_id=abcde to the end of the normal url you have copied with the LaunchFlows Direct Checkout Link Generator. For example:
Where “your-custom-checkout-page” is the url of the checkout and “xxxxx” is the main id of the variable product and “abcde” is the id of the variation you want to send to the checkout.
You may optionally specify an initial quantity as well by adding this to the end of the string.