Y'know, I'm not very excited about most mail that I receive; are you?
There is an exception though, and that is bulky mail or mail with some "thing" in it.
Those I find irresistable; it's a bit like Christmas, isn't it?
3D mail packages are exciting to the addressee, and are vastly underplayed. Somethings to consider:
Hone your target down to definite prospects; users of your service(s) and/or product(s);
Get your name on the thingy you are enclosing, and keep it relevant to what you offer; make your company memorable by it;
Avail more if they wish upon a visit or what have you;
Test against a non 3D mailing; you will be stunned at the difference in response*
(*Keep in mind that creative contributes about 20% to the overall response, the balance coming from the list and the offer.)
So give it a try and let me know the results.