It's fairly hard to uniquely fingerprint iOS devices, there aren't that many configuration options, most people use the stock browser etc.

Actually I suppose this may have changed since the introduction of safari extensions on ios? I haven't looked into this since that released

We considered doing something like this but it would be very hard to ensure no false matches which could result in security compromises.

We also considered placing the session id in the url for the checkout, but again decided against due to security concerns.