Is it possible to send a message to a bitcoin address? how? Say I have an online store that accepts bitcoins and want people to be able to contact me through my wallet, because that is ultimately the entity involved in an hypothetical trade.

Can that be done? how?
Not in any way that is intended by the protocol.

What do you mean by "ultimately the entity involved in a hypothetical trade"? It's just a hash of a public key, the corresponding private key to which you hold.

What is preventing you from having a contact form or e-mail address in the online store?