Overview
Invoice templates are custom HTML, editable under Settings > Invoice Templates, filled in using Handlebars{{variable}} placeholders. The variables below are the ones documented and validated for the invoice template scope — writing a placeholder that isn’t on this list is rejected when you save the template.
Seller and Purchaser contact fields (email, CC, BCC, phone) are resolved from Contact groups, not from the entity directly — see that page for how a contact is chosen when more than one could apply.