/**
 * Validates the billinig tag.
 *
 * It must be 4–16 characters long and contain only alphanumeric ASCII characters [A-Za-z0-9].
 *
 * @param tag The string that represents the billing tag.
 * @return The billing tag if it's valid. Otherwise, throws an error.
 */
export declare function validateBillingTag(tag: string): string;