Plan Management - Adding Zoom Whiteboard Plan

I have recently purchased an additional Zoom One Pro license to which I would like to add on the "Zoom Whiteboard" plan - so that there are unlimited whiteboards for this particular license

However when I go to buy another Whiteboard License, it says "Your account has custom pricing. To make changes to your account, please contact us."

I have contacted Zoom regarding this but I've not heard back yet.

Thanks

Tom