Important: If you will be using the Twenty Nineteen theme included with WordPress 5.0 or if you will be using PHP 7.3, you should also be using WooCommerce 3.5.2+. In this release we’ve added the necessary styling for stores to look nice in the Twenty Nineteen theme and made backwards-compatible code tweaks to prevent notices and warnings when running PHP 7.3.
Payment gateway software is usually delivered pre-integrated as part of an e-commerce platform. This type of software can also be used separately by companies who do not want to limit their options to the integrations provided by e-commerce vendors. While most providers of e-commerce platforms offer integration with the most popular payment gateways, other options may be preferable, especially for markets that aren't well served by the main players.