@NotANumbersGal
Entering you inventory/COGS on a monthly basis is completely fine and an accepted method in the industry. Just know that your financial statements (P&L and balance sheet primarily) will only be accurate as of the date of those entries. For example, if you enter sales, ending inventory and COGS on the last day of each month, your reports will only be accurate as of those dates. You won't be able to run an accurate report as of October 15, for example.
Google "periodic inventory system" and "retail method of inventory costing". You will need a starting point for your inventory which should be the last physical inventory. Then, estimate the ending inventory for the month using the retail method of inventory costing. Alternatively, if you have an established history of knowing your cost of goods sold %, you can use that to establish your cost of goods sold for each period end.
It may seem a little confusing at first (it did for me at least) but after studying it and doing it a few times, it will become easier. This is great stuff to know if you're going to be entering EOM numbers. I would venture a guess that almost all bookkeepers do not know how to do this so you'll be way ahead. Good luck and feel free to ask any further questions.
