If the code is based on the first phase of Freda’s Growth Obsession arc in Goddess Maker, in which she uses the lab’s debt as leverage in order to coerce the lab to make her bigger, then I assume the mechanic is to hold a number of height serums and/or super height serums in stock prior to reaching the day.
I don’t know if the reservation mechanic was translated to this fan mod, as that would help in allotting what the lab sets aside for Freda and what the lab keeps to sell and for personal use/experimentation.