Hey @craig-martin - I did a bit of digging into this on our end, and from what you’re describing there it seems like it should be working, so it’s odd that the issue is popping up for you. One thing to double check though is to see if the customer attached to the cart you’re creating is assigned to a specific company location: Headless with B2B
If you’ve confirmed that, just ping me here in this thread and I can take another look (If you can share the shop ID/location ID and customer ID, I can do some more digging on my end to investigate further).
Hope this helps a little bit though.