You can currently switch between templates by editing the Invoice, then navigating to Invoice Details, where you'll find an option to select the desired template.
To make this more user-friendly, we’re implementing an enhancement based on your suggestion: We’ll be adding a template selection dropdown directly in the Send popup. This dropdown will be pre-filled with the template chosen in Invoice Details, but you'll have the flexibility to switch to a different one before sending.
We appreciate your input and will keep you posted once the update goes live.