Customer: "Hi, we need to add a discount system to our site as it's Black Friday on Friday"

This is now the fourth one this week. Are they taking the piss?

Seems like you're a magnet for stupid clients. Even if you could pull it off on such short notice (for a price), do they really want to enable an untested discount system? Something will break and they'll have thousands of orders with wrong discounts that will need to be cancelled and pissed off customers.