You're trying to get to balance. Import Certificates trade cheaply once a country is in balance or surplus, they offer little subsidy to exports at that point. The policy choice becomes level of balanced integration, rather than a competition for trade surplus.

(balanced global trade integration)

(i guess this isn't quite right, because Import Certificates force balance immediately. you're trying to rearrange the economy so that trade would be in balance without tariff or subsidy. once that happens, import certificates become cheap.)