FYI - I've been using WooCommerce for many years, before they were known to the broader market.
They now offer Splitit on their platform, this is excellent, not hyped but solid news.
WooCommerce are a solid business with many clients.
Like many have said, it's as simple as a plugin, takes less than a few hours to install, test and deploy.
Here's the plugin link -
https://wordpress.org/plugins/splitit-installment-payments-enabler/#description
