Incentives for international studies

Hope this is not going off topic as it is somewhat going into the area of shopping advice...

But, we are conducting some remote usability testing on an international product.

For this testing we have participants from a variety of countries and have a set incentive of £x amazon vouchers for each.

A problem exists however in that amazon vouchers are not transferable between countries. If we buy a UK amazon voucher this cannot be cashed in in Japan. Only on UK amazon which is less than ideal, many items will not ship internationally.

Amazon is out. So a question.

What is a good way to provide incentives internationally?

Is there any way we can provide a voucher (straight cash transfer is a bit vulgar and breaks rules) that is valid no matter where the participant is?