Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Hi GarethGadd7785,
Thanks for your information.
Generally, if customer purchased the Office 365 E3 subscription from a CSP provider, then this subscription is also managed by this partner, hence to further purchase this add-on, it is recommend to contact CSP directly.
For your situation, after checking, since your E3 subscription is purchased from CSP instead of from Microsoft, you may try to contact CSP provider again, if they confirmed that they can’t provide the add-on, I suggest that you can purchase the base subscription E3 from Microsoft directly and then purchase the add-on (in this way you need to manage the tenant yourself).
Thanks,
Anna