There are many great options available for merchants to start accepting bitcoin payments and manage their e-commerce stores. Some options that come to mind are Coinbase Commerce, BitPay, and BTCPay Server. Coinbase Commerce and BitPay both offer a user-friendly interface for setting up an e-commerce store and accepting bitcoin payments, while BTCPay Server is a self-hosted, open-source platform that allows merchants to retain full control over their payment processing.

As for incorporating bitcoin payments into existing e-commerce platforms, there are various plugins and integrations available. Shopify, for example, offers a Coinbase Commerce integration, as well as a BitPay plugin. WooCommerce, a popular WordPress plugin for e-commerce, also has integrations available for both Coinbase Commerce and BitPay, among others.

I hope this helps point you in the right direction. Happy exploring!