Can't send invoice to customer if it's a draw against a retainer

I just created an invoice against a retainer. I noticed that its status was immediately “Paid”. And, when I look in the list of Invoices, it’s way down on the bottom with all the other paid invoices. Now, what feels broken to me is that this invoice was never actually sent to the customer. So, I now want to send the customer the invoice along with a comment that it’s a draw against the retainer and that no payment is due. However, Harvest doesn’t seem to have any feature that makes it easy for me to send this sort of notification. In fact, since Harvest automagically marks the email as paid (and I can’t unmark it as paid) there’s no way for me to “Send Invoice”. All I can do is “Send Thank-You”. I don’t really want to send a Thank-You note to my customer for a payment — they didn’t make a payment. I generated an invoice for which no payment is due. The worklow seems a bit broken to me.