With variable products, the syntax to send any particular variation directly to your custom checkout requires you to change the id from the main product to the variation id instead.
You need to change the id of the main variation product to instead use the id of the variation you want to send to the checkout
You can also create an Instant Simple Product Bundle by using multiple variation id’s separated by a comma. See More Info Here.
/your-custom-checkout-page/?add-to-cart=xxxx where “xxxx” equals the id of the variation
You may optionally specify an initial quantity as well by adding this to the end of the string.