Did you see the Flight of the conchords "femident" commercial episode?
You definitely should engage a lawyer to negotiate this for you, and what you choose to do should hinge on what that piece of music means to you. frequently in advertising the deal is what is known as a buyout, which means that they want all of the writers and Publishers share of the performance royalties and the right to use the music exclusively. Depending on the piece of music and the fee I may have different feelings about it. writing music for film and TV I face similar situations where the composer fee and agreement also involves the ceding of part or all of the publishers share of performance royalties. I look at this aspect of the music I make as disposable in a sense. If I were asked to write somthing say in the style of "peter bjorn and john" specifically for a commercial and the deal was a buyout and the fee respectable (more than $10k USD) I'd feel the same. But if I was approached to have one of my songs used in a commercial and seeing that all of my songs are the greatest songs in the world ever, and I plan on using them myself as a recording artist, I would think differently about this. Chances are a decent attorney can sway the agency to accept a license of your song. I have frequently been persuasive enough in deals I negotiated, and have had similar experiences in deals negotiated by my attorney , to retain 50-75% of my publishers share on television productions, and do commercials as licenses.