It would appear that as zen calculates to 4 decimal places

The 40% off price is 0.0360 each
It is shown rounded to 2 decimal places in the cart ie 0.04
0.0360 x 250 = $9.00 which is correct

0.04 x 250 = $10.00 is an approximation and incorrect

It would appear that users with such low value items might need to show them to 4 decimal places instead of 2 decimal places.

or price them as 1 item = (packet of 50) for $3.00
Then the discounts will calculate better for 2 decimal places.