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?
There are dozens of us 😩