Hello,

We are using a Shopify headless build and are developing a custom Back in Stock feature on the frontend and have had success setting it for out of stock products based on the following guide:

https://developers.klaviyo.com/en/docs/how-to-set-up-custom-back-in-stock

We have no problems making a call to a Product that contains both a Product ID and a Variant (for instance: size or colour) — if the product is published in the catalog, it creates that entry under Profile > Email address as well as shows in the activity feed.

But for simple products without variants (flat with no options), we are struggling to figure out what to include — sending just the product ID but no Variant ends up with an error stating that the “variant” property is required. Most guides seem to just use the Product ID for the Variant, but that does not seem to register despite a successful post request being made. We ensured the said item in question is published but cannot find it anywhere despite a successful 200 response.

Also there seem to be a few different URL endpoints being used: `https://a.klaviyo.com/api/v1/catalog/subscribe` and `https://a.klaviyo.com/onsite/components/back-in-stock/subscribe` — does it matter which one to use?

Any help at this stage would be greatly appreciated, thanks in advance!