No continue shopping button, just a check out button.
When a customer buys an item update an information line on the bottom that tells them how many items they have and what the current total is. When they finish they choose the [Check Out] button and at this point they get a line by line list of what they purchased and have the ability to adjust quantities or delete some of the items. If they do change an item to zero quantity though I would like it if the item stayed on the screen so they can change their mind back again. I have this in real world all the time where a customer overspends so they start juggling product until they get to a price they're comfortable with. Just refresh the screen after every change so they can see where they are now.
This way there is no interruption of their momentum. The other thing this does is make it theirs for just those few minutes and people tend to come back and get it later.
Better to buy twenty and put 5 back then buy 4 and quit due to getting tired of the back and forth.
It's all about the money!!!
