I'm a bit puzzled by the question. Posters are one medium for presenting your research. Other options include talks (or the associated slides), articles and abstracts. A poster only really makes sense in a poster session, when there is actual space for putting it up, and ideally some time has been allocated for authors standing next to their posters and presenting them to interested people.
Sending in a poster as part of an application in general doesn't seem to make much sense, just like sending in the slides to a talk you have given. It's not really the right medium for the purpose. And the same principle applies to your presence before the selection committee, unless they are doing a poster session.
You should prepare a presentation of your work in a suitable format (probably an abstract, maybe in somewhat extended form, along with a URL to a preprint or published article) and send that along with your application. You can still bring your poster in case there is a situation in which you can use it. And slides in case you get the opportunity for a little talk. Don't think of any of these things as a magic bullet just because someone has complimented you on it. Choose the right medium for the occasion; if you have material in a less optimal medium such as (presumably) your postposter, keep it in your sleeve as an ace to be presented in case an appropriate occasion should arise.
In my opinion, a poster would have to be truly extraordinary in a way that cannot be captured in another medium to justify sending it with your application. Besides, I am not even sure how you would send it. By snailmail would be slow, unusual and a hassle. If you send it electronically, most recipients probably wouldn't know how to print it in an appropriate size, and reading it on the screen is probably not much fun.