Charge a card for a cart

Returns the **tax rate** (a fraction, e.g. `0.07` = 7%) for a ship-to address, computed with our own engine - the same combined state + county + city + district rate (statutory VAT outside the US) that the post-payment verifier uses. **How to use it** 1. Send the buyer's `shipping_address` (address only - no items or amounts). 2. We return `tax_rate`. 3. Compute `tax = round(tax_rate × taxable_base)`, where the base is the items subtotal (net of discounts) **+** shipping, only where the destination taxes freight. 4. Display the computed tax to the buyer as its own labeled line item (for example "Tax" or "VAT") in the checkout totals before payment - do not fold it silently into the grand total. The buyer must be able to see the amount they are being charged as tax. 5. Charge the buyer, then send that same tax back on the order in `merchant_supplied_totals.tax_cents`. Because the quote and the post-payment verification run the **same engine**, charging this number means **no tax-shortfall correction** is applied to your payout. A tax-free destination returns `tax_rate: 0` - collect no tax. `override_enabled` tells you whether PeptidesPayment expects to apply this tax for you: `false` means your store is configured to compute its own tax, so leave your own tax in place and ignore `tax_rate`. It is `true` for every other store (including tax-free destinations, where `tax_rate` is `0`).