It can be important to remember that with the Retail Express integration you could have a profile that does NOT have marketing email subscription accepted 

If needed, you can add the following flow filter to your flows to filter out profiles who haven’t explicitly opted in (i.e., Retail Express only profiles): 
If someone can or cannot receive marketing  > Person can receive email marketing > Because person subscribed.

Active profile not subscribed
If a profile is marked as "active" but not subscribed, it may have been added through general engagement, such as Retail Express without subscribing. These profiles have a "Never Subscribed" status, indicated by a yellow checkmark, and can receive certain emails like abandoned cart flows. However, exercise caution and ensure compliance with local regulations when contacting them. They cannot receive SMS or mobile push messages until they opt-in.
https://help.klaviyo.com/hc/en-us/articles/115005246968

If needed, you can add the following flow filter to your flows to filter out profiles who haven’t explicitly opted in (i.e., general engagement profiles): 
If someone can or cannot receive marketing  > Person can receive email marketing > Because person subscribed.