3? You must be assuming I meant a person who bought 2 ships to begin with. I'm just talking about any old run of the mill swindler, who buys one good ship knowing it is good and with intent to swindle DST by exploiting this alleged replacement program. He buys the good ship, he sends in its nacelles, he gets a replacement. Now he has 2 ships minus one pair of nacelles. I can think of all kinds of reasons why a less-than-noble person might *want* the extra parts -- as you said, customs. The saucer is kind of a standalone toy, anyway.
As you say, the question for the swindler would be, is it cheaper to buy another $40-50 ship to get the desired extra parts, or to swindle DST? That depends on the cost of this alleged replacement process. Sending in the nacelles would cost a few bucks at most. If DST makes you pay for shipping for the new one, that could be $10-15 or so... So swindlers can essentially buy a nacelle-less ship for $15-20, at great cost to DST as they made no income at all from it.
At least if they made us send in the saucer only, that would still be cheaper for us than mailing them the whole ship (it's pretty light) and they could at least verify that the audio distortion is present so they would know we were a person with a genuinely defective product, and not just a swindler.