It's all one thing. The "bundle" is just things that cannot be added through the compendium: the art pack and the puzzles are just content from the book that are presented in a way that makes them accessible to Roll20 users, not really "bonus" content. For example, the book comes with puzzles, but the puzzles are not useful in Roll20 if viewed on a compendium page, so they are included with the book as an add-on so that the puzzles can actually be used. Roll20 doesn't offer books in a standard pdf-style format. They have been converted so that they actually are usable by Roll20. So you can read through content page by page, but when it comes to things like spells and monsters, they are presented in a way that they can be dragged and dropped into a game. Similarly, a book might contain maps, but no adventure content (the DMG and Volo's Guide are examples of this), so Roll20 presents that content as an add-on so that the maps can be used in a game, instead of just looked at on a compendium page. The only thing that might be considered "bonus content" are the token and art packs, which Roll20 includes to add a little value to the purchase. If purchase of a book on Roll20 were just a readable on-line version, they would not be very useful. The price as presented is just the price of the book. Some older books have had a price reduction, Tasha's has not, at this point. TL;DR: The "bundle" is just a way of presenting the full contents of the books in a way that is useful on Roll20.
