The shipping_cost object embedded in a Stripe Tax calculation or transaction.
Reachable from LatticeStripe.Tax.Calculation.t/0 and
LatticeStripe.Tax.Transaction.t/0. Carries the shipping amount alongside its own
tax_breakdown, because shipping is taxed separately from line items in most
jurisdictions.
Embedded value struct: fields are additive. Keys Stripe adds later appear under
:extra rather than being dropped, so a new field never breaks decoding.