I answered this in an email, but here are the answers for anyone else who is interested.
1) To get the total of the shopping list to go back to zero after you have deleted the items in it, on the Items page of the Shopping List click on "Lowest Unit Price" and then the total will be zero.
I.E. you need to click on something to have the List recalculate the total.
2) The Shopping List functionality is to be used to determine which store your list of items will be cheapest at, hence unit prices are used. If the actual prices were used, you wouldn't be able to tell which store the shopping list would be cheapest at.
Should I add a button to calculate the total price? Could I assume that you always wanted to buy the quantity that is stored for that price?