Is there a standardized way to express discounts or credits in an order summary?
Example...
Sub total: $20
Discount: -$10
Total: $10
I've seen discounts expressed as -$10, $-10, ($10), and +$10.
Are any of these expressions more "correct" than the other? Is there a common or standardized way to express discounts? Where would these standards be documented?
tl;dr
"-$10" is about as clear as it gets.
Ask yourself how most people write out a subtraction problem?
The discount in your example is "subtract ten dollars".
Write it like any normal person would: $20 -$10 = $10
($10) is for accountants. Normal people don't write their numbers that way. And an accountant will have no trouble figuring out -$10.
+$10 makes no sense whatsoever: "$20 + ($10 discount) = $10" is really going to make users scratch their head.
$-10 is just weird. Is there a standard where denomination comes before the operator?
Prototype a simple checkout workflow and see if it confuses anyone. Then you'll have some data before you throw it out in the wild.
One way to consider something as "standardized" is to look at how the major players in a space do something -- i.e, users expect from you the same experience that they get from popular sites/apps.
With that in mind, I went on Amazon.com and applied a coupon code.
Amazon expresses discounts as -$10
I would try a few options and test them with your stakeholders. Shopify did some good research and has their policies for handling this situation well documented. Here you can see how they apply discounts for a variety of situations. https://shopify.dev/themes/pricing-payments/discounts
Since this question was first posted, Nielsen Norman Group published an article on the UX of applying discounts. Per their research:
Our study participants often had difficulty understanding what was meant when a charge was listed in a line item and then later subtracted in a separate line item. It was clearer to users if the item was simply listed as free.
This statement was specifically for an item that was wholly free after the discount, but one might argue that listing several items and their corresponding discounts could become visually messy and complicated.
An alternative could be to show the updated total price for each discounted item, with some information on the promotion outside of the total amount column. By showing only positive amounts, it's easier for users to scan and estimate totals in their heads. You can also put the total savings somewhere on the screen before the final checkout button.
