Actually, since the 3rd mailbox is just a audio message, is does not need a mailbox, it can be an OGM thats play the message and then hangs-up.
For a easily changeable audio inventory you could do:
OGM: inv1 > plays your voice saying "todays inventory is, slippers"
inv2* > (select advaced>voice synthesis of script) in script/comments type: %slippers%
inv3 > play voice "high quality shoes"
inv4* > voice synthesis of script > %shoes%
inv5 > play voice "thanks for the preference, gerard!"
Then in inv1 > advanced > is "set local variables" type: slippers=12&shoes=9
*optionaly inv2 and 4 could play your own voice too, you would need to record 1 wav file for each number from 1 to 50(?), set in advanced > play extrnal file: x:\xxx\%slippers%.wav, then when variable slippers=12 the ogm 2 would play the file 12.wav.
Even if your IVM is the Small Biz (wich is expensive, for 20 dollars extra you get 70 OGM) you can play the hole inventory with 2 OGM, using a php plug-in.