It is better for a few people to get away with "cheating" than for relief and reconstruction to be delayed -- or made prohibitively expensive -- for thousands of people, and for the whole economy to be damaged. There will always be a few people who "cheat." You don't damage the economy of a whole country simply to make sure that this does not occur.
Simple cost benefit analysis is all that is necessary here.
The money lost by putting a moratorium on ALL customs duties for a limited period of time will be far less than the amount of income that will come into and circulate within the economy from the quick rebuilding.
The whole country suffers if tourism is delayed because of the domino impact of people financially unable to rebuild or repair. The people whose homes are destroyed are the people who work in the tourism industry. The people whose boats are damaged or destroyed are essential to the tourism industry.
Then there is the MORAL issue: There is no way that taxing people for the privilege of rebuilding their homes and businesses, let alone taxing people who DONATE RELIEF SUPPLIES can be morally justified.
Put the customs people who may be laid off to work doing something else...like claims adjusting, for instance. There are other things they can do to earn their salary.