Hi, Peter. There is nothing wrong with posting links here to products or sites, including commercial sites, that might solve another poster's problem or answer his or her question. Hundreds of such links are posted in Apple Discussions every day. I probably post five or ten a day myself. In the present case, you can certainly post a link to the Moshi keyboard skin if you wish.
It is
not permissible to post a link to your own web site or to a product or vendor if the link doesn't have any direct bearing on a previous poster's question or add pertinent information to the ensuing discussion. And if you will receive any kind of compensation as a result of others visiting a site you've linked, you must disclose that.