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QB uses average cost for inventory valuation, FIFO which is what QBO uses is just average cost per purchase still the item cost is average cost.
If you buy 5ea for $25, then average cost is easy, $25 / 5 = $5 each So yes you sell one and the average cost of $5 posts to COGS
Intuit programmers dropped the ball though.
buy 3ea for $10 and the average cost is $3.3333333
but costs is rounded to two digits, so when you sell one, 3.33 is posted to COGS
when only one is in stock, the average cost is .03333
sell it and 3.33 is posted,
3 * 3.333 = 9.999
10 original cost less 9.999 = .001 or some other amount due to rounding left over
over time that can add up, and there is no way to know what purchase(s) is causing the issue, there is no way to adjust item cost either.