@suerobbins you shouldn’t need to pay to use h5p with WordPress – we certainly don’t! But it might depend on what kind of WordPress site you are using. There’s two kinds: the WordPress.com site is essentially a blog with very limited functionality. What you need is to host your own WordPress site, this will allow you to install whatever plugins you want, including H5P.
You can do this for relatively cheap through various companies that offer WordPress hosting, such as Siteground. If you go to this page of their site, you can set up a WordPress site relatively cheaply (about £3 a month). You should then be able to use that to install the h5p plugin and host the content.
If you want a one-to-one meeting about this to explore the options, I’m happy to do so.