Buy offsets or removals now, charged to the account's saved card

Charges the account's saved card for a carbon purchase, one-time or monthly (the first month now; WooCommerce Subscriptions renews it monthly on the same card). Available only after the account switched API purchasing on in My Account → API (403 api_purchasing_off otherwise); the account needs a saved card on the product's rail — PayPal for offsets, Stripe for removals (409 rail_not_covered). Exactly one of carbon_kg, spend_usd or items. The order buys whole units of the product (a kilogram, or a tonne) at the ALL-IN product quote, with the web order's processing fee; a spend_usd purchase can exceed the budget by less than one kilogram's price, and a product sold by the tonne is bought by quantity (422 buy_by_quantity). Idempotency-Key is required. A decline is final: post again under a new key. A purchase is a final sale.