Re Certificate Untrustworthy site

this message came to me from a client- does anyone have this issue please?
"Whatever site you are using to host the invoice has an invalid security certificate. Is it possible for you to just attach it and email it to me?"
Lubos, is it possible for you to send the certificate authentication re the emails as the message does come up about a certificate being required for untrustworthy sites (something to that affect) It has obviously bothered a client and was wondering if there is actually a certificate that can be produced to put his mind at rest? Many thanks, Jan

I would suggest in the meantime to simply send the invoice as an attachment PDF to that particular client instead of the link to view / download from the online site. When you email there is a tick box allowing you to choose how you wish to email the invoice.