Like Internet peering agreements, the UPU model only made sense when there was about the same amount of traffic going in each direction. In that case, it makes

Like Internet peering agreements, the UPU model only made sense when there was about the same amount of traffic going in each direction. In that case, it makes sense for each country to just pick up the cost of domestic last-mile delivery, because other countries are doing the same for its outbound packages. The problem is when there's a continuous structural trade imbalance, and more packages flow one direction than in the other. Then you have a de facto subsidy scheme, and it makes sense to terminate or renegotiate the agreement and switch to something more along the lines of sender-pays.