We've already responded to your emal inquiry about this matter, but here was our reply in case anyone else may be reading along here:
What you describe unfortunately seems to be an endemic issue with Yahoo!Mail that we see cropping up from time to time. Messages sent from some parts of the Internet will be delayed seemingly at random while other messages get through promptly, or there may even be an overall delay on all messages for a while; normally the delayed messages do eventually show up, so it's usually just a matter of waiting.
If you get a complaint from a buyer @yahoo.com saying that they have not received their link, you can go to your Seller Admin > View/Download Transaction Log, and find their Transaction ID. We link each of those IDs to the Common Thank-you Page for that order, so you can right-click on a buyer's Transaction ID link (or control-click on a Mac) and select Copy Link Location/Address/Target/Shortcut (the exact wording varies by browser), then paste that link into your reply to the buyer. Hopefully, your own mailserver's region of the Internet won't be affected by the same delays affecting mail traffic from our region of the 'Net at that moment, or your own reply may be just as delayed as the thank-you message we originally sent the buyer.
Once our system sends a message, it's out of our hands to do anything more to affect its delivery or even see what happened to it once the receiving mail server accepts the message; that's just the nature of how email works, so we have no way of knowing how often Yahoo!Mail may occasionally delay delivery of our messages or for how long. We only know what sellers have reported to us in the past, and what we've found by manually sending test messages to one of our own addresses @yahoo.com and seeing how long they take to arrive. Test messages we've sent while delays are known to be happening have arrived anywhere from a few minutes to hours late, but at least they do typically arrive eventually. While other email providers can be affected by delivery delays of their own, those cases are infrequent, and we really only see this happening as a frequently recurring problem with recipients @yahoo.com in particular.