Thanks, that's very helpful already! And yes, good call, I have the 3" single of WHATEVER and indeed it is obiless (it could have a had very cut tiny obi, but alas!). Same for ayu-mi-x II version Non-Stop Mega Mix and ayu-mi-x III AOV, which don't have one either. ayu-mi-x for me does though. So versions of A BEST and A BALLADS with obi/without slipcase do exist? (I've only had my own forever so can't imagine them any different, haha)
Perhaps we can keep track of a master list in this thread. I only know of slipcases/no obis for the three releases I have that have then, perhaps there are more. I'll start the list below with the suggestions by ~angel*ayumi~, elaborate upon it as you wish:
Singles
poker face (3")
YOU (3")
Trust (3")
For My Dear... (3")
Depend on you (3")
WHATEVER (3")
LOVE~Destiny~ (3")
TO BE (3")
Boys & Girls
kanariya
Fly high
H (special commemorative version for 1 million copies sold)
Mirrorcle World (special version)
Days / GREEN (special version)
Albums and compilations
ayu-mi-x II version Acoustic Orchestra
ayu-mi-x II version JPN
ayu-mi-x II version US+EU
ayu-mi-x II version Non-Stop Mega Mix
ayu-mi-x III Acoustic Orchestra Version
ayu-mi-x III Non-Stop Mega Mix Version
BEST (first edition with slipcase)
BALLADS (first edition with slipcase)
Love songs (which version(s)?)