How do I add an optional item to invoice?

[waiterock]

I built an invoice to send to clients so they can pay their non-refundable holiday boarding deposits for my kennel.  How do I add an optional trip cancellation insurance that they can pay if they want to get it back in case they have to cancel?

[Minion]

I do not believe that there is a way to add an optional service/item to an invoice that would be.  You could add said item to the invoice, and then create a contract/service agreement that would include details about receiving the insurance back if they have to cancel