10% discount calculating wrong amount

[harry]

I set-up a 10% discount code. The product price, before discount, is $194.75. 10% off should yield a $19.48 discount. But it's only calculating $18.45.



This is an "all items" percentage discount. FYI, it's a quantity of 205, per-item price of $.95.

[E-junkie_Guru]

The reason for the discrepancy is explained on this help page:

http://www.e-junkie.com/ej/trouble.discounts.discrepancy.htm



Normally, that would result in a discrepancy of maybe a few cents, but the same rounding imprecision applied across 205 items is magnifying the discrepancy in your case. A 10% discount amounting to $19.475 off the order total, spread across 205 items, works out to a discount of $0.095 off the price of each item, but we can't pass individual item prices to checkout with sub-cent precision, so that gets rounded to $0.09 off per item, times 205 items = $18.45 total discount.



If this isn't reflecting an actual or likely order, and you're just testing the discount function, try making a dummy product with whatever price you wish, that you can Add to Cart to test with the discount code.

[jason2]

Can this be looked at again? This seems to be more complicated than it needs to be to end up with a wrong number.

It looks like the math is the following:
1. Get the pre-discount cart item total * the percent discount
(so in the original example, it would be $194.75 * .10 = 19.475)
2. Take the result from number 1 and divide it by the quantity to get the discount per item, truncated to two decimal places if the third decimal is 5 or lower, or round to two decimal places if the third decimal is greater than 5
(19.475/205 = .095 truncated to .09)
3. Take the result from #2 and multiply it back by the quantity to get a different answer from #1
(so .09*205) = $18.45

Why are steps 2 and 3 necessary? If it's necessary to calculate the discount per item, is the rounding correct? Why is .095 truncated to .09 and not rounded to .1?

[E-junkie_Guru]

To expand on our help page about this, PayPal in particular doesn't allow us to pass any whole-order total or discount amounts to checkout; they only allow us to pass individual item prices (and tax/shipping amounts), so we have to distribute any Cart Item Total discount amount across the price(s) of all items in the buyer's cart.

As for why partial-cent decimal remainders get truncated rather than properly rounded, I think that's just a matter of our database fields for prices only being able to handle up to 2 decimal places, so any additional decimal places just get dropped.