Support for allocation method on product percentage discounts

[limoniapps]

Function behavior v automatic discount behavior
A function-based product percentage discount is always applied using the allocation method "across".
This is in fact not aligned with how standard automatic discounts work.
A standard automatic product percentage discount uses the allocation method "each". This is also true for a standard discount code.

Impact of the allocation method
This is important because using the wrong method results in a subtly different discount amount due to the different rounding strategies.

• "each": the discount per item is calculated, then floored, then multiplied by the quantity
• "across": the discount of the entire line is calculated, then floored

Example
Assume a customer adds 7 items to the cart of a product variant that is sold for $67.96.
Assume a discount function applies a 10% product discount to this line (resulting in allocation method "across").
The discounted price for 7 items will be $67.96 x 7 = $475.72 - ($475.72 x 0.1 = $47.572, which is floored to $47.57) = $428.15
If the same 10% discount is applied using an automatic discount (resulting in allocation method "each"), the the discounted price for 7 items will be $67.96 - ($67.96 x 0.1 = $6.796, which is floored to $6.79) = $61.17, multiplied by 7 = $428.19
The difference between both calculation methods is 4 cents.

Proposed solution
Developers should have control over which allocation method should be applied.
The default method should be aligned with the way standard percentage discounts work, i.e. they should use allocation method "each".

[limoniapps]

I notice this discussion has been closed. Can you share with us if/how this will be handled?

[jharlap]

Apologies, I didn't mean to close it, especially with no comment!

It seems like the issue at hand is less about allocation method and more about control over rounding. Rounding to the smallest unit of the relevant currency is often a source of a small amount of error in the discount amount, unfortunately. It's not clear to me why you would want to amplify that error - that's certainly never our intent. Can you elaborate on what the impact of the current rounding behaviour is to either you, as an app developer, or to a merchant that uses your app?

[limoniapps]

Hi there @jharlap,

My intent is simply to ensure function based discounts behave exactly as native Shopify discounts do when it comes to rounding. I would argue that applying a 10% product discount using a Shopify discount code should yield the exact same result as a function based 10% product discount.

Currently rounding works differently for function discounts compared to discount codes and rounding in draft orders is different again. It doesn't feel unified.

[jharlap]

Understood and I agree that this isn't optimal - we definitely want to create a consistent experience across the platform. I've logged this to our internal bug tracker. I have no timeline I can share regarding when this would be resolved, however.