50 pack can be had a bit cheaper on Amazon S&S if you're paying postage on this deal.
I thought 'certified plastic free' was just for the plastic-looking packaging that holds the tea pyramids (the one inside the cardboard box). Apparently it's made of a tree fiber they made clear. I had no idea of the tea pyramids are actually plastic free (until reading the link you posted - thanks!)
Free delivery over £10 orders.
I stopped buying teapigs due to that report of up to 15 Billion plastic particulates in each cup. Turns out they are certified plastic free now: https://www.teapigs.co.uk/blogs/news/plastic-free-trust-mark