I am assuming that you mean that you have "blank shirts" as a "part" and you "build / complete" the final shirt design version part" prior to shipment?
You can think of this as a good example of simple manufacturing - parts, sub assemblies, final product assembly.
In some ways, it isn't so different than the idea of making a sandwich. It takes an inventory of hamburgers, buns, pickles, etc to make the final product. (the sandwich)
Amazingly, as common as this concept is - it usually takes specialized inventory control software to manage it.
AFAIK, neither medium level accounting software like quicken nor square can deal with it correctly.
What I do is make a guess at what will sell and put some of it up as inventory for one "final product / item" and some of it up as another "final product / item".
I deal with the build time by using the delivery time.
It is clunky and I could be wrong but I don't think that square can do more than this.
