Order-line item price computation with 0 decimals and tax inclusive price settings.

[Sidsector9]

We are facing an edge case problem while rendering the filtered order-line item prices.

The problem: Aggregation bias caused by error propagation due to rounding with 0 decimal numbers.

We are using the WooCommerce Deposits plugin.

---

In the following scenario where:

1. A product has a 50% deposit plan applied
2. The number of decimal setting is set to 0

On the block Cart page, the order-line item of a product with deposit applied is incorrect.

The Deposits extension makes use of the rest_request_after_callbacks filter hook to filter the order-line item subtotal. For example like the following:

If you observe the image above, the sum of the prices in red is higher than the price in green.

In the callback for rest_request_after_callbacks, Deposit sets the prices in red as following:

$deposit_amount_including_tax = wc_get_price_including_tax( 
 	$cart_item['data'], 
 	array( 
 		'qty'   => $cart_item['quantity'], 
 		'price' => $cart_item_deposit_amount, 
 	) 
 ); 
  
$data['items'][ $key ]['totals']->line_subtotal     = $money_formatter->format( $deposit_amount_excluding_tax ); 
$data['items'][ $key ]['totals']->line_subtotal_tax = $money_formatter->format( $deposit_amount_including_tax - $deposit_amount_excluding_tax ); 

Because the decimal setting is set to 0, $money_formatter->format() rounds up the prices if the decimal is >= 0.5 and rounds down if the decimal is < 0.5 . So this is what is happening:

At the same time, $data['items'][ $key ]['totals']->line_subtotal and $data['items'][ $key ]['totals']->line_subtotal_tax cannot be set to floating numbers as only the integer part is considered by Woo Core on the client side:

woocommerce/plugins/woocommerce-blocks/assets/js/base/components/cart-checkout/cart-line-items-table/cart-line-item-row.tsx

Lines 152 to 155 in 7fe00de

	let lineSubtotal = parseInt( totals.line_subtotal, 10 );
	if ( getSetting( 'displayCartPricesIncludingTax', false ) ) {
	lineSubtotal += parseInt( totals.line_subtotal_tax, 10 );
	}

Would you have any advice to best handle a situation like this?

[senadir]

Would doing $money_formatter->format( $deposit_amount_including_tax ) - $money_formatter->format( $deposit_amount_excluding_tax ); works?

[Sidsector9]

@senadir Thanks for looking into this. Your solution will work for the prices given in this example but will fail when the product prices are $7.75 or $13.5.

[senadir]

The product price can never be 13.5 because WooCommerce will round it at save.

[Sidsector9]

@senadir apologies for the delay, I was on leave.

The product price can never be 13.5 because WooCommerce will round it at save.

During the computation of total it does not consider the rounded values. I have tried with multiple values and found the issue to be consistent.