Europe isn't that big, and shipping expenses shouldn't be very high for a relatively small parcel. It'd be required that participants tried to arrange the waiting list in order of proximity to make shipping cost lower. So the parcel could travel from US to UK or Iceland then going to the next closer participant.
You'd just need to set the rules, trial term, checking participants identity and good will, etc. Just like you'd do within the US but across the Pond.