I contacted Afterpay… this is what they said:
"At the moment, we’re unable to integrate with Wix. Having said that, we’re always looking at ways that we can integrate with new merchants. Disclosing why we aren’t able to currently integrate with Wix has to do with the way our plugin works, which I’m unable to do. I know it’s very frustrating and hopefully we’re able to integrate with Wix in future. "
We have been waiting a long time for Wix to integrate with Afterpay… doesn’t seem like its happening anytime soon.