I have a question about commission

Okay here is my situation...

I'm currently developing and selling games/applications for blackberry smarthphones. I use a few different websites to sell my applications. Each website takes 20-40% commission.

Now is that 20-40% commission considered a business expense? If so could I write it off? I've asked a few people (non accountants, but self employed) and I get yes and no's all the time.

Does anyone know if this is considered a business expense. I personally think it is because they are taking 20-40% of my revenue from my product, in return they bring me more sales. I see it no different then advertising really.

Thanks

20 to 40% on blackberry games? hhmmmmmm