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.
QuickBooks uses FIFO for inventory valuation while erplain uses the Moving Average Cost.
In erplain, Purchase price and Cost are two different numbers. The Purchase price is the default price being used when creating a Purchase Order. The product Cost refers to the cost to acquire or create a product and will vary each time you add products to your inventory.
erplain uses the Moving Average Cost (MAC) method to calculate the product cost and inventory valuation. The MAC of each product is calculated after every Purchase Order (except drop shipping), Stock Entry or when creating a new product (Initial cost).
The MAC is not synced with QuickBooks. This applies to products and bundles (assemblies and kits).
You can find additional information on how QuickBooks calculates the Cost of Good Sold on this page.