Quote - This reminds me of the story of a (very well known, by I forgot his name, sorry) artist who just send a reminder to the publisher that he still had not seen any payments for the copyrights etc, so perhaps they could give it a look in their administration. And by the way, you have some extra's to offer as well.

 

This positive, non-escaling but firm approach landed the guy a life-long relationship with that publisher. Publishers don't want to escalate, they don't want law-suits for a book-cover, they want good book covers without trouble. You can give them that.

My message: artists should not invest in negative energy. Go for the positive twist.

All the best

The story of Ray Bradbury and EC Comics springs to mind.

From wikipedia:

Adaptations of Ray Bradbury science-fiction stories, which appeared in two dozen EC comics starting in 1952. It began inauspiciously, with an incident in which Feldstein and Gaines plagiarized two of Bradbury's stories and combined them into a single tale. Learning of the story, Bradbury sent a note praising them, while remarking that he had "inadvertently" not yet received his payment for their use. EC sent a check and negotiated a productive series of Bradbury adaptations.[9]

Friendly but firm and professional is probably the best way to approach it...

...my tuppence worth.