Flat charges

Some levies are not a percentage of anything. Colorado's Retail Delivery Fee is $0.31; Minnesota's is $0.50. A TaxRate is a percentage and refuses to be anything else — deliberately, because that refusal is load-bearing elsewhere — so a fixed amount needs its own seam rather than a rate faked from the order total.

Nothing is shipped. These levies are per-jurisdiction, move on their own schedule, and no authoritative compilation of them sits behind this package. The defaults (NoFlatCharges, NoOrderFlatCharges) say so plainly, and a host that knows its own obligations binds a source.

Two seams, because there are two kinds

The distinction is not cosmetic. A per-delivery fee run through the per-supply seam is charged once per line, so a two-line order pays $0.62 for one delivery. No care inside a per-supply source can prevent that: it is handed one line at a time and cannot see that the lines share a delivery.

So within assessOrder() the per-supply seam is not consulted at all — the document's own source decides. A standalone assess() is a one-line transaction, where the per-supply seam is exactly right and behaves as it always has.

A per-delivery fee

use Cbox\Tax\Contracts\OrderFlatChargeSource;
use Cbox\Tax\Enums\{JurisdictionLevel, TaxTreatment};
use Cbox\Tax\ValueObjects\{FlatCharge, OrderAssessment, TaxOrder};

class RetailDeliveryFees implements OrderFlatChargeSource
{
    public function chargesFor(TaxOrder $order, OrderAssessment $assessment): array
    {
        // Due on a delivery that contains taxable goods — which is why the source
        // is handed the finished assessment and not just the order.
        foreach ($assessment->assessments() as $line) {
            if ($line->treatment === TaxTreatment::Standard) {
                return [new FlatCharge(
                    code: 'co_retail_delivery_fee',
                    name: 'Retail Delivery Fee',
                    amount: Money::of('0.31', 'USD'),
                    level: JurisdictionLevel::State,
                )];
            }
        }

        return [];
    }
}

Bind it in a service provider:

$this->app->singleton(OrderFlatChargeSource::class, RetailDeliveryFees::class);

Where the money lands

gross stays net + tax — that invariant holds throughout the engine and several things depend on it — so charges are added beside it, never folded in:

$assessment->gross();       // the exact sum of the lines' net + tax
$assessment->chargesTotal(); // document-level charges the buyer is billed
$assessment->payable();      // gross + line charges + document charges

Set passedToBuyer: false for a levy the seller must absorb by statute. It is still reported — you owe it — but it is excluded from chargesTotal() and payable(), because it is not something the buyer owes.