Urgent help please :( When I fill out a stock card using the both Weighted Average Cost and FIFO method, Perpetual Inventory System.

All transactions exclude GST but 20% GST needs to added to the purchase cost listed for all credit purchases for Accounts Payable valuation purposes.

There is a purchase return transection, returned for credit to supplier.

Should I include GST for returns as well ? since I added GST on all the purchase?

Also when I have a transection 15units damages and written off (no particular date) at stock take on FIFO method stock card.

I write it to purchase section "-15", "unit cost (the oldest inventory from Balance unit cost)" , "-value" is this correct ?

Thank you tutors, I appreciate your help.