Custom wordpress plugin for selling music

I need a custom plugin developed for wordpress for selling music online. I am currently using music store plugin [login to view URL] What I need is an enhanced plugin which might have shopping cart and would allow additional payment gateways. Unfortunately, the music store plugin doesn't have shopping cart and it does not support any other gateway other than paypal. The gateway details shall be reviled to the selected developer. Kindly explain in your bid how you intend to develop this plugin with a list of features and also give a basic description of how plugins are created in wordpress. No bids shall be considered without these requirements. You can either enhance the music store plugin or develop your own plugin with all required features or if you already have something matching my requirement, I am willing to buy it too. I am an expert level wordpress plugin developer my self but I am short on time so I am outsourcing this plugin development. So please value your and my time and do not place generic bids! My max budget is $150 so do not bid for more than that.