How should I word a WhatsApp order confirmation message so it stays a utility message?

Keep it transactional: confirm the order, the total and the time, and add nothing promotional. A promotion makes Meta re-categorise it as marketing.

• Utility templates are for transactional messages, typically sent in response to a customer's action, such as an order confirmation or update.
• If a utility template contains marketing material, it is automatically re-categorized as a marketing template.
• Send offers as a separate message, not inside the confirmation.
• A template has an optional header, a body up to 1024 characters, an optional footer up to 60 characters and up to 10 buttons.