90% discount - how does it work?

You have to be able to afford the item normally, and it can't be a cube package (or package in general?). When you purchase the item, you'll have an option to click "Use Coupon" or "Use Discount" or something like that right beside the price of the item in the pop-up.

Not true. I was able to buy the black market merchant 30 day one (priced at 30k) for 3k, and I only originaly have 12k loaded.

But you are right about cube packages.