gorgeous bag!! definitely inquire about a discount for the pen mark though, especially if it came from a dept.store. I find that dpt. stores have a very liberal policy on discounts for damages on products they sell. Also tell the manager that you definitely won't be returning it if you get a discount.
A girl in one of the other handbag forums purchased a mj stam for 10% off final sale at NM because the closure had a dent or something. So I don't know about 20% off but 10% definitely,
good luck!