Removing ads is a one-time fee as stated in the OP. There’s a separate subscription for additional features but it’s not required. You’re free to disagree with the pricing but try to understand the facts before complaining.

As a separate point to the dev, since it’s not just one or two people getting confused, perhaps a link to the one-off ad removal on the Sync Ultra page if it’s not there already?

Plenty of people in the UK/Europe use third party apps already, iMessage is certainly less of a big deal than it is in the US.