Wordwall now says that if the tax in your area changes, your renewal bill changes too — and they don't consider that a price change. That wording matters: any notice or consent rules that would normally apply to a price increase won't kick in for a tax change. So the amount taken from your card on renewal can be higher than last time with no warning. The underlying tax is set by your government, not by Wordwall, which limits how much extra they can take this way, but you still bear the difference.
“Taxes are applied to every payment, including renewals, based on the rules in your location at the time. This means the tax on a renewal may differ from your first payment, changing the total charged. A change in tax is not a change to the price.”
What you can do — Check the exact total on your renewal receipt rather than assuming it matches your first payment, and cancel before the renewal date if the new total isn't worth it to you.