Every time I create a custom or reserved listing the customer has this issue. The link they have you send is a Share and Save link, which may or may not be the problem... but what I've found is that it is not in their system, yet. I have a larger shop, so even regular listings take a little time to show up for purchase, but I suspect the share and save link takes a while to be in the system as well no matter what size the shop happens to be. Most of my customers can purchase later, though. I vaguely remember a post a while back saying it could take up to 15 minutes for a listing to show up in the Etsy system, but I've noticed it can take longer!
I've been holding off for at least 15 minutes to send the link to a customer, but when I do that, it takes them a while to come back and buy. Some come back and say never mind, which is why it's horrible that it takes so long for the link to work! However, if a customer is going to not want the item after waiting 15 minutes to a half hour, you probably avoided a bigger issue down the line anyway.
I wanted to add real quick that I rarely use the app and usually the desktop, so it's problematic everywhere.
