Suppose there are tiered discount rates: say $10\%$ up to $5000$, $15\%$ above $5000.$ Now suppose I need things that cost $b$ before any discount is applied. Assuming $b < 5000$, I get the 10% discount, having to pay $90\% \times b$. Could I perhaps add some more items to be eligible for the 15% discount and still end up paying less than $90\% \times b$?
If I add an item worth $x$ such that $b+x = 5000$ then the discounted amount will be $85\%\times 5000 = 4250$. I will save money if this is less than the original $90\%\times b$. $$ 4250 < 90\%\times b \iff b > \frac{4250}{0.9} = 4722.\bar{2}\ .$$ This inequality means that for initial (undiscounted) amounts of $4722.22$ or higher, adding exactly enough to reach 5000 will give me more stuff while costing less than the original.