Hi @AliceC,
Thanks for sharing this question with us.
Indeed, you can add a custom property to this dropdown, generally speaking.
However, we can only segment off of “first-level” data, that is the any non-nested data coming from the Placed Order event. The first-level data options will pre-populate when you click on the “choose property” dropdown. Because of this, product_type (or any other nested data in this event) won't appear as a filter option in flows or segments. Even if you type it in, the segment builder won’t be able to “recognize” that nested data.
Rather than using product_type, consider tagging these particular items with “Skirt” in Shopify and segment using the “Ordered Product” event which will allow you to sort by specific tags associated with the product. Or, you can send us a custom Placed Order event (using the Track API) including the product_type data point as first-level data so we can segment off of this directly.
I hope that’s helpful.