Sorry, kids-r-fun, but all the etiquette mavens agree that there is NO polite way to "direct" folks how to give gifts, or even to refer to gifts in an invitation, period. It is not even correct to say, "no gifts, please" because gifts are never supposed to be expected.
There was a huge discussion on one of my other boards about a bride who had instructed her bridesmaid (who was throwing a shower for her) to include a separate sheet in the invitation giving her gift registry info. The bridesmaid balked, and posted her problem on our board for discussion. We all agreed that would be tacky, tacky, tacky.
This info CAN be circulated, but it has to be done in an informal way--word-of-mouth or telephone. If you or the best friend normally exchange emails with some of the people on your invite list (and I assume you would) you could casually mention it there. Word should get around.
Just, please, I beg of you, DON'T put it in writing anywhere in the invitation, even on a separate card.