@john john Thanks for the confirmation. In that case the billing is correct should be correct for the premium service bus as it is a dedicated resource and would be charged per hour billing (fixed pricing) and not based on the usage. In case if you have any concern about your current bill then I will suggest you to engage us on the billing support request here so it can be reviewed further.
If you don't need the premium namespace, then I will suggest you to delete the premium namespace ASAP to stop further billing.
To add more why the pricing for Service Bus Premium is higher than the Standard tier because it offers additional features and capabilities that are designed for mission-critical applications.
The Premium tier provides resource isolation at the CPU and memory level so that each customer workload runs in isolation. This resource container is called a messaging unit. Each premium namespace is allocated at least one messaging unit. You can purchase 1, 2, 4, 8 or 16 messaging units for each Service Bus Premium namespace. A single workload or entity can span multiple messaging units and the number of messaging units can be changed at will. The result is predictable and repeatable performance for your Service Bus-based solution.
Not only is this performance more predictable and available, but it's also faster. With Premium Messaging, peak performance is much faster than with the Standard tier
In summary, the additional features and capabilities of the Premium tier come at a higher cost, but they are designed to meet the needs of enterprise environments that require high throughput, predictable performance, and support for advanced networking.