ESchwartz hit the proverbial answer on the nailhead and you will probably be good to go with the changing of the text to the field name and then adding the the :.3 modifier. And the suggestion to accommodate empty #purchase1 fields was also something to consider.
I have a similar situation but I DO use a shortened created column for the field that I want to be two characters long ...
I have a #genre column created from my #tagesgm column that I want to be two characters long: {#tagsgm:re( *(.[^\,]?)[^\,]*?(\,|$),\1\2)}
I believe adding a \3 behind the two and substituting your base created column of #purchase1 would work. I then use the following plugboard:
[{#readrank:ifempty(9)}{#genre:ifempty(SF)}] {#stripped_series:lookup(.\s,#initials,.,#shortene d,series)}{series_index:|-|-}{title}
As you can see, I use {#genre:ifempty(SF)} to create a default value so that it's never EMPTY, which MIGHT be something you want to consider.
However, I think the earlier answer is PROBABLY what you were looking for and I only offer up my solution as another way to approach the possibility of handling a blank #purchase1.
Again, all of this stuff from me is actually only regurgitated Chaley. Any added errors are all mine.
