Hi Tom,
Thank you for your reply. I will try the meta key for the GTINs.
If I want to add a filter in functions.php like the brand one you pasted above and the meta key I want to use to get the brand name is wpfoof-brand what would the snippet look like?
I would really like to be able to also create two more structured data attributes – material and colour. If I use the meta keys for material and colour from woocommerce pa_colour and pa_material what snippets can I use to create structured data attributes for color and material that will be schema friendly and follow these guidelines https://schema.org/material and https://schema.org/color ?
Thank you in advance. I will buy the unlimited sites license if I can get these working even though I will only use it on one website.