Rounding differences potentially lead to order failure

[luzat]

Describe the Bug

Rounding differences can change the basket to contain physical goods and lead to additional errors such as order failure.

To Reproduce

1. Create an order with only digital goods which introduces rounding differences (I guess because PayPal is receiving net values; here, this happens with a VAT of 19% and two items of 9.50 gross value each).
2. An extra_line item Subtotal mismatch will be added by default by PurchaseUnitSanitizer. This item is a physical product.
3. An error can occur at a later point, which would otherwise not happen, such as in OrderProcessor::process:

if ($this->order_helper->contains_physical_goods($order) && !$this->order_is_ready_for_process($order)) {
    throw new Exception(__('The payment is not ready for processing yet.', 'woocommerce-paypal-payments'));
}

This is because $this->order_helper->contains_physical_goods($order) now returns true.

Expected Behavior

I expect the order to go through.

Actual Behavior

An exception is sometimes (always?) thrown after the order is completed and the order fails (even though it has already been paid) at the mentioned lines in OrderProcessor::process.

Environment

• WordPress Version 6.8.1
• WooCommerce Version 9.8.4
• Plugin Version 3.0.5

Additional Details

My workaround is to patch PurchaseUnitSanitizer::sanitize_item_amount_mismatch:

$this->purchase_unit['items'][] = (new Item($line_name, $roundings_money, 1, category: Item::DIGITAL_GOODS))->to_array();

Problems with that implementation: This will always add digital goods and is only PHP 8.0+ (not 7.4).