An email to itch.io support will take care of the issue. Ive had a few problems with itch not sending links. If they payment actually cleared and went through you can likely get it sent again from using the Recover Purchases function.
I contacted itch.Io through their customer service and I used the buy-back function more than once yesterday, but they just wrote that their earnings were processed, but they didn't do anything for a day, so I can wait to send you what I paid for.
Unfortunately, contacting itch.io support is based on priority as far as responding in a timely manner. It could be a matter of hours or a matter of days depending on what it actually was, and with the holidays possibly longer.
Moderators on the support board will tell you the same thing.
