How can you investigate eCard delivery problems reported by site users?

Sarah Calatayud 2

We've set up a Mother's Day eCard donation option and received a complaint from a site user who says she purchased an eCard and scheduled delivery to a recipient, and the recipient never received the eCard. How do I even begin to investigate what might have gone wrong? We will have to reimburse her for the failure, but I would like to be able to tell her where things went wrong. She's quite disappointed.