The easiest way to add a payment method to your WordPress site is by using a payment gateway plugin. There are many payment gateway plugins available in the WordPress repository, and the most popular ones are WooCommerce, PayPal, Stripe, and Square.
WooCommerce is a free and open-source plugin that allows you to sell products and services online and accept payments. It supports various payment methods, including credit/debit cards, PayPal, and bank transfers. It also has a robust inventory management system, shipping options, and tax calculation features.