Items on Amazon regularly slip in and out of availability for seemingly arbitrary reasons. It's not a great way to determine what's in print -- particularly with items from smaller publishers.
Volume 7 of The Complete Peanuts is still listed on Fantagraphics' webpage, both as a single volume and as a slipcase with volume 8.
Slipcases are five buck a piece through Fantagraphics directly.
~C
"everybody in this room is wearing a uniform, don't kid yourself" -fz