[fjtorres]

e-bundle is a nice, short-n-snappy way of describing what you're trying to do.
You might even want to bill them as "catch-up e-bundles" so you can list them as companions to the new release.
And if you want to provide a single-file download of discrete volumes to take advantage of the psychology of "many for one price" there is always the zipped folder, which would let you throw in promos like high-res cover images for wallpaper, a newsletter, an anthology of sample chapters of other similar authors, etc). And that would also take care of the multiple sku issue.
Lots of tricks can be played once you start thinking of the bundle as a promo tool.

Which reminds me that in the old days many publishers *used* to do all sorts of consumer-level promotions, newsletters, grab-bag bundling, etc. Stuff like the Ace doubles... Or DAW numbering their releases...

With ebooks lowering entry barriers, maybe we'll see a return of creative book retailing.

Good luck!