At the end of each calendar month (usually around day 1-3 of the following month so that all items from the previous month are recorded as posted) I take a Paid and Posted report and download that and keep it in Excel. This shows the total sales income (inc postage)  for the month. This total figure I plug into an Excel sheet each month.

In a separate tab on a Excel sheet I keep a track of all postage expenses (and others) incurred.

Among all my other various business expenses (stock cost inclusive) it is pretty simple to deduce a profit/loss summary.

It's this summary info that I present each tax year to my accountant.

At the end of each financial year we also do a stock count and value that at cost to us.

All this info my accountant can work with to produce our tax returns to the ATO each year.

We are not big enough yet to register for GST so that complication is not in the equation yet.