For our customers that connect erplain to QuickBooks Online, we recommend to use QuickBooks exclusively for your inventory valuation.
QuickBooks is your accounting system and it uses the transactions sent from erplain (Invoices and Purchase orders) to calculate product cost and inventory valuation. erplain doesn't sync cost data with QuickBooks directly.
QuickBooks uses FIFO for inventory valuation while erplain uses the Moving Average Cost.
You can find additional information on how QuickBooks calculates the Cost of Good Sold on this page.