The gateway plugins are free; the processing fees are not – and that’s true everywhere.
The official Stripe and PayPal gateways for WooCommerce cost nothing to install, and WooCommerce charges no platform fee on top of them. WooPayments (WooCommerce’s own gateway) is also free to install.
What you always pay is the payment processor’s per-transaction fee – Stripe, PayPal or whoever moves the money takes their cut of each sale. That’s not a WooCommerce charge; it’s the cost of accepting card payments on any platform, hosted or self-hosted.
The distinction that matters: hosted platforms like Shopify often add their own fee on top of the processor’s unless you use their in-house payments. WooCommerce with the official gateways doesn’t – so at volume, you keep more of every sale. More: Stripe’s fees · vs Shopify