Create hosted checkout link

Creates a short-lived hosted checkout URL for a package, one domain, or up to 50 domain registrations. Use package_id with cycle_id for an existing package, domain with years for one domain, or domains for multiple domain registrations. callback_url is required. customer_id is optional; when supplied, it must reference a customer account (account_type 2). The checkout link is cached for 30 minutes. When the customer opens it, BillingServ creates a checkout-only session, allows supported saved payment methods, and redirects back to callback_url after payment. For package options, price, fee, and billing cycle are always resolved from the selected database option value; supplied options.amount and options.cycle_type values are accepted for request compatibility but do not override catalogue data.