Here are some guidelines how to send us the songs you want us to mix. We will talk not only about the format of the tracks but also about organization. Organization is actually the key to a good workflow for any job in the world, and since mixing is actually “organizing” the separate elements into one coherent whole, it's even more important for us to receive your data in a structured way.
We don't want to lose time finding out which drum parts belong to which song. (although it might create some unexpected nice results if we use parts of different songs to make a new one, but that's not exactly what we want here)