Upload/download and payment environment.

I'm an online mixing and mastering service, clients send me their audio tracks for me to mix and master them. Normally a client pays afterwards, but i'm really done with waiting till the client likes to pay after 2 or 3 months. So here is my idea to make life for me and the client much simpler. I want an online environments where the client can login with his own login and password. In there, i would like to have everything managed. The client can upload his files to my VPS, can see his invoices and do the payments, can listen to (non downloadable??) preview mixes and can download the mastermix after the final payment. I'm already working with Buckaroo, so it needs to work with Buckaroo.