Apart from the two options you have provided, I could see this...
- EA admins with write permissions can directly purchase savings plans from Cost Management + Billing > Savings plan. No subscription-specific permissions are needed.
Partners can use Home > Savings plan in the Azure portal to purchase savings plans on behalf of their customers. The Partners can also buy an Azure savings plan through Partner Center. Partners now have the flexibility to purchase Azure savings plans through the Partner Center portal, APIs, or continue using the Azure portal.