In theory this should work with Universal Analytics, at least for logged in users ( I assume that your users are logged in if they want to buy).
You can pass a client id as a parameter when you create the tracker. The client id is supposed to be formatted as UUID, so you'd have to store that along with your real client id in you backend system and pass it in to the tracker as a part of the confuguration json object (optional third parameter in ga create). Apparently this get retroactively applied to the running session (no written source for that but I recently attented a conference where a google employee said as much, so I assume this is legit).
So as far as it concerns data collection UA is ready for multidevice. I frankly do not know to what extent this already works in the Analytics Interface.
I recently had a glimpse at a Analytics Premium Account which already had some new multdevice reports. I don't know if the fact that those reports are, at least for the moment, absent from the free version means that multidevice tracking does not work yet on the free version (those reports are along the line of Venn diagramms for "How many users used more than one device" and the like).