For segmentation to work correctly it's best to export ALL products, forget about exporting just a specific collection, and then create segments INSIDE that specific collection. For technical reasons related to the feed specification that won't work.
Here's a workflow that should work for you:
1. Choose the "all" collection with your Flexify Advanced settings page.
2. Choose which collection you'd like to appear in your Facebook catalog:
YOURDOMAIN.com/collections/...
3. Take the names of the collections you want your Facebook catalog to include and fill out the "Collections" field (no spaces between the names, only comma-separated, please).
4. Select which custom_label field you want to use (for example "custom_label_0").
5. Regenerate the feed with your Flexify Overview page by clicking on the "Create feed" button.
6. Open your Facebook Commerce manager, select your Facebook catalog, and go to Catalogue > Sets. To create a new Product Set, press the 'Create Set' menu and select 'Use filters'.
7. Create the product set using a "contains" rule of "custom_label_0".
8. Copy the "name_of_collection" (e.g. 'facebook-shops'), paste it in the field, and hit enter without waiting for autocomplete.
NB! Important tips regarding using the Product Segments feature are available in our help articles. Also, please find more info about how to Choose products to show in a collection ad.