I am obviuosly going to pay the full whack and if perchance I have to miss one month that is my problem.
My point is that most of us are not that organised ( Read; We have family or work committments that make 3 months off a long way away.
I am sure that we don't have to use a whole bottle of flavour at once we could therefore say:
Announce the date about 2 weeks in advance:
"First testing dates are 1/2/3 of a given month ( 3 days )"
People sign up for that date as they are reasonably confident of attending. They have a last minute backup as do the people who know they can't make that date, For them we hold another 3 day course at a later convienient date?
I also assume these flavours have a reasonable shelf life? So there is no great time pressure!
Hope this helps
Will