/**
 * Returns the list of country codes that are valid billing countries for an offer.
 *
 * Mirrors sma-checkout `billingCountryOptions()` logic (options.ts:21):
 *   - `bundle` / `insider_print` offers → unrestricted (all countries)
 *   - EEA offer country               → price-zone countries ∪ all EEA countries
 *   - Otherwise                        → price-zone countries only
 *
 * **Intentional deviation from sma-checkout:**
 * The source (`get-zone-by-country.ts`) throws `Error('No price zone found for country …')`
 * when the offer country is not in any price zone. This function returns all countries instead.
 * Rationale: Web Components have no React error boundary — an uncaught throw would crash
 * the host page silently. Returning all countries is the safe degradation: the Apple Pay
 * session itself validates the billing address, so no invalid payment can succeed.
 *
 * @param offerCountry - ISO 3166-1 alpha-2 country code of the offer (e.g. `'GB'`)
 * @param offerVariant - Product variant string (e.g. `'digital'`, `'bundle'`)
 */
export declare const getSupportedBillingCountries: (offerCountry: string, offerVariant: string) => string[];