> But be aware - study the depth of some of these plugins. Invoice entry screens are fraught with all sorts of things like real time validation (e.g. will you allow a user to specify a quantity of an item greater than what is in stock? Can you calculate tax amounts on the fly line by line? Can the user change the pricing of the item on the fly and are there rules that have to be adhered to in order to prevent fraud) etc.

Yes. You hit the nail on it's head. All of these apply in this case.