I once ordered some parachute cord and was sent the wrong color. The seller told me they would send out the right color, and to keep the cord they'd mistakenly sent since it wouldn't be worth the return shipping.
OTOH, I know of a pocket knife manufacture who will often request that product complaints be sent back to them so they can investigate the problem to determine if any changes in production need to be made, and most always back the knife with replacement. This company strives to develop a good relationship with their end line user, even to the point of including us in design sometimes.
I had a knife from Browning (don't know how to put the trademark symbol in here) that lost a single screw. Most companies would have simply sent me a few screws to fix myself, but Browning had me sent back the whole knife, and replaced it with a brand new one. You see, that knife was made in Japan, and Browning probably didn't stock any parts for it, so they just opted to replace it, not wanting to get into parts inventory for a line they only licensed.
Because of this, I can understand them wanting you to send back the kit (show the manufacture). OTOH, it still puts the burden on you to ship something to them, when it really isn't your job. OTOH, if the end line user doesn't help to make things better (in this case by providing the product that is improperly assembled), then I can see how it would make it more difficult to identify and solve the problem.
Since you did not order this kit, I can see no reason for them to require you to send it back, however if it's a request to help them solve the problem on their end, then I that I can understand.
I guess it all depends on how much it's worth to you to help them with this problem. What kind of relationship do you have/want with them. They should send you what you ordered regardless. If you don't feel satisfied, decide what it will take to satisfy you, and then talk to the people in charge to see if they can work to insure your customer loyalty.