Hi Bee,
I never give discounts that I'm already set up to give anyway. Every now and then, I'll have someone ask for a discount on one item because they can't afford to buy it for gift giving or some other story along those lines, and while I appreciate that they took the time to reach out, my prices are firm. I set my prices at the point that it makes it worth it for me and I treat it like any other professional business.
Bulk discounts are one thing and something I would have thought out prior to their asking, so I would just offer it right off the bat. However, $5/18 = more than 25%! That's very unreasonable, and I don't know how high you set your profit margin to be, but for me that would be a significant chunk/probably more than the margin itself after labour, fees and material costs.