Copywriter direct marketing

I am new to this so it is difficult to be specific. I will be selling disc/dvd sets online and by direct mail so will need, initially, a long form sales letter. I dont know whether this sort of job is hourly or by contract. My mentor tells me that the length will be about 20 pages and will include pictures. The disc sets will be variable but are basically how to make money selling online. The sets can be from 3 to 20 discs. There will be front end and back end products so other letters will be needed but in a short form