p.s. answered questions are below.

FLOW WHEN VALUATING STOCK TO STANDARD PRICE
Cost Price product (in product form) = 9 EUR
Product purchase (in purchase form or invoice) = 10 EUR, Tax = 19% =1,90 EUR
Product sale = 30 EUR, tax = 19% = 3,80 EUR
Stock location configured with account 3000

Receive the products
account 3000 = D 9,00 ← Type balance – Stock account, defined in location
account 1510 = C 9,00 ← Type balance – Interim account, Goods received, awaiting invoice
				(Account 1510 can been seen as addition to 1500 – Creditors)

Receive the purchase invoice
Note that creditor has added shippingcost line to the invoice. Shipping cost (or other purchase cost) should account choosen manually by the accounting user on the invoice.
account 1510 = D 9,00
account 7095 = D 1,00 ← Type P/L – Price differences creditor (also called discounts creditor) should be a parameter.
account 7065 = D 10,00 ← Shipping costs purchase (excl. VAT)
account 1612 = D 3,80 ← Type balance – Vat account to receive
account 1500 = C 23,80 ← Type balance – Creditor account

Deliver product
account 3000 = C 9,00 Stock account
account 1310 = D 9,00 Type balance – Interim account, Goods send, to invoice
			(Account 1310 can been seen as addition to 1300 – Debtors)

Invoice customer
account 1310 = C 9,00
account 7000 = D 9,00 Type P/L – Cost of sales

account 8000 = C 30,00 Type P/L Sales account
account 1602 = C 5,70 Type balance – Vat account to pay
account 1300 = D 35,70 Type balance Debtor account

FLOW WHEN VALUATING STOCK TO AVARAGE PRICE
Cost Price product (in product form) = 9,00 EUR for delivery 1 – 7,00 EUR for delivery 2
Product sale = 30 EUR, tax = 19% = 3,80 EUR
Stock location configured with account 3000

Receive the products delivery 1
account 3000 = D 9,00 ← Type balance – Stock account, defined in location
account 1510 = C 9,00 ← Type balance – Interim account, Goods received, awaiting invoice
				(Account 1510 can been seen as addition to 1500 – Creditors)
Receive the products delivery 2
account 3000 = D 7,00 ← Type balance – Stock account, defined in location
account 1510 = C 7,00 ← Type balance – Interim account, Goods received, awaiting invoice

Receive the purchase invoice 1
Note that creditor has added shippingcost line to the invoice. Shipping cost (or other purchase cost) should account choosen manually by the accounting user on the invoice.
account 1510 = D 9,00
account 7065 = D 10,00 ← Shipping costs purchase (excl. VAT)
account 1612 = D 3,61 ← Type balance – Vat account to receive
account 1500 = C 22,61 ← Type balance – Creditor account

Receive the purchase invoice 2
Note that creditor has added shippingcost line to the invoice. Shipping cost (or other purchase cost) should account choosen manually by the accounting user on the invoice.
account 1510 = D 7,00
account 7065 = D 10,00 ← Shipping costs purchase (excl. VAT)
account 1612 = D 3,23 ← Type balance – Vat account to receive
account 1500 = C 20,23 ← Type balance – Creditor account

Deliver product
account 3000 = C 8,00 Stock account  (avarage price)
account 1310 = D 8,00 Type balance – Interim account, Goods send, to invoice

Invoice customer
account 1310 = C 8,00 (Avarage price from order or when manual invoicing from stock)
account 7000 = D 8,00 Type P/L – Cost of sales

account 8000 = C 30,00 Type P/L Sales account
account 1602 = C 3,57 Type balance – Vat account to pay
account 1300 = D 33,57 Type balance Debtor account


Question: I think stock transactions must be based on the cost price and purchase entries on the purchase price. Am I right ?
Answer: Yes, to my knowledge. See also example above.

Question: Do you sometimes inject costs in the product's cost ? Like deliveries ?
Answer: Deliveries are mostly seperated at invoice line. The this is not a onjected cost. But indirect kost for purchasing, like department cost or location cost are sometimes injected in de cost price of the product. As I haven't seen this kind of functionality in OpenERP today, I will administrate these cost as normal cost, and devide them before year end closing.
This injected cost would be a nice to have for feature developments.

Question: Can we use the same interim account for both receive and deliver operations. (that should simplify a lot our changes to do) ? Instead of 2800 and 2801, just 2800 ?
Answer: I see your point, but no it can't. I had setup this in first circumstance for simplicity, but now I used in the examples the right accounts with explanation. These accounts are registered in addition to the creditor and debtor account. If we would use the same, the balance would be not correct.

Question: If the products return to your warehouse and you have to generate a a refund customer invoice, what are the entries ?
Answer: exact the opposite bookinglines

Question: I still have a doubt on something: some accounting software do not manage stock operations (purely accounting, no warehouse management system). In that case, how do these companies manage stock entries ? I think they manually do an inventory and related accouting entry at the end of the year. Do you know how this entry looks like ?
Answer: To my knowledge 2 situations exists then:
1.They do exactly the same but not real time, but in batches e.g. At  end of month. Uses paper sheets to register all warehouse transactions and compare and combine them with the invoices.
2.They do not care about warehouse volume, but register only financial value. End of year a stock counting/valuation will done to correct value.

All answers are generated based on my implementation- and accounting experiences with several ERP- and accounting software and are verified with book “Boekhouden geboekstaafd” from Hoogheid and Fuchs used at higher economic eductions in the Netherlands.

Thank you for your commitment.