Definitely record the dollar amount you are purchasing as this figure is given as expenses to your accountant.

I also keep an Excel spreadsheet which is super handy come tax time.

In a nutshell, I -

Record each months gross sales

Record each months' purchases of fabric/stock etc

Run a transaction report each month to get the fixed and variable fees.

Of course, items that I purchase this month may not sell until later in the year, but I just tally it all up and give to my agent.