Hi jacqueline0630,
I understand how you want to send an email with different templates for different customers. Currently, this isn't possible.
QuickBooks Desktop uses the default template when sending invoices to customers.
I'll pass this along to our product team to let them know of your business needs.
Here's an article for more additional information if you want to personalize emails to customers and vendors: Create Custom Email Templates In QuickBooks Desktop.
Keep me posted if you need anything else. I'd be happy to help.