A few countries I'll be visiting this summer still sell International Reply Coupons. It might be interesting to pick some up and see how difficult it is to exchange them. Would a PostNL point even know what to do with one?
I agree that UPU members are required to accept them. PostNL closed all of their post offices a few years ago in favor of contracting out to businesses, who may not be familiar with them.
It looks like Omniva (aka/fka Eesti Post) also ships worldwide and charges in Euro.