Here are some of the popular OpenCart Payment Gateways Extensions.
1. Stripe Payment Gateway. Developed by ClearThinking
This Payment Gateway is OpenCart 2.0 compatible and also the official Stripe integration for OpenCart. Quick checkout option using the Stripe checkout pop-up implemented with option to disable.
2. Checkout by Amazon (USA) – Standard. Developed by Integrio
Customers can use their existing Amazon.com account to checkout from your OpenCart store. In order to use this module, you need SSL certificate. Here is the checkout process. Customer will be redirected to the Amazon website for verification and confirmed back to your store. The shipping address will be culled from the customers Amazon.com shipping address. Sandbox mode is supported if you want to test the module prior to going live.
3. Braintree Payment Gateway. Developed by ClearThinking
Braintree is owned by eBay and this extension allows you to charge subscriptions. Includes support for storing multiple credit cards for easier transactions for repeat customers.
4. PayPal Payments Advanced. Developed by Integrio
OpenCart comes built-in with PayPal Standard, but your customer must finish the transaction outside of your website. By using this PayPal Payments Advanced extension, customer can pay for the goods without leaving your online store. This service does cost $5 a month.
5. Easy OpenCart Stripe Payment Module. Developed by BenW
This OpenCart Stripe extension an inexpensive alternative to the one developed by ClearThinking, but without frequent updates. No popup window implemented and not compatible with OpenCart 2.0 as of this writing. To see the extension in action, you can also watch my video here.