You can have an invoice template for a particular client without affecting the default template for the rest of the system. However, each client can only have one template applied.
- Click on the blue Clients tab
- Click on the desired client
- Click on the Invoice Presets tab
- Click on the desired template and the green check mark will bounce to this template
The change will automatically update for any invoices for this client. Changing the template for the client will override the template used under the Settings so if you change the template under Settings, this client will not be affected.