The more Zylon Coins you buy at one transaction, there's bonus coins.
Let's say you bought 5,000 + 500 zylon coins for 50$.
You buy a 1 year membership for 4,000 zylon coins. You get 1,500 zylon coins left.
Then, you buy 3 months membership for 1,250 zylon coins. You get 250 zylon coins left.
You normally get 1 year membership for 40$ and 12.50$ for 3 months membership. Total of 52.50$.
I'd say -1, but it'd be awesome if it gets implemented.