Thanks for your feedback regarding a plug-in for Affinity Publisher, Bill. This is indeed a great idea and something we'll look into. In the meantime, we've put together a help center article for Affinity Publisher users who'd like to make use of our PDF Uploader for their files. It covers best practices and common mistakes for Affinity Publisher-created files.
I also encourage you to contact Affinity directly to let them know you'd love to see a Blurb plug-in for Affinity Publisher.