There is at least one plugin that allows WordPress sites running WooCommerce (a very popular and free e-commerce solution) to accept Bitcoin. As far as I know no similar solution exists for Monero. How difficult would it be to develop such a plugin? Is Monero core development at such a stage that developing a Monero Payments plugin is not yet feasible or advisable?
Serhack is developing the woocommerce plugin for wordpress. Check it: https://github.com/monero-integrations/monerowp
I don't know of a plugin put there are a few merchant solutions available. See the "Libraries and Helpers" section on the following page: