I want to add an optional donation (not vatable) to the subscription (vatable) invoices I produce. This would be like a service charge on a restaurant bill.

Is there a way to do this in Zoho Books so that if the customer pays the invoice without the optional donation I wouldn't have to produce a credit note.

Basically I want the subscription fee to be recorded as a debt but not the optional donation. The optional donation would only be recorded if it was paid.

Thanks