Desing Problem with Store Transactions and Balance !!

Hello,
that's a common problem in my designs, and every time I think a lot about it and now I want to know what to do as a Standard rule in all my future designs >

Assume I have a Store of Items, I've a Purchase Orders and Receive Invoices, and I have two ways to get the balance/Credit:
1- I Usually don't make a transaction table, I just make a Views to collect data and calculate the In/Out Items.
cons: It would calculate all the PurchaseOrders and RecieveInvoices every time I want to know the balance.

2-Making a Transaction table with a field of the Last Balance.
cons: I should handle the Insert/Edit/Delete operations on Transaction table every time the happens on the PurchaseOrders or RecieveInvoice !!

what is the best way? or What is the wrong in my designs !?

thanks
w8ing your advice...