I agree, that should be the way to do it. Have the system set to "shoot" the bar code and have the user enter the quantity.
Note, there are several issues to keep in mind:

1/ you may need to think about serial numbers that may need to get captured using a wand as part of the transaction.
2/ you may need to capture a sales person's code before completing a transaction ie . is the clerk performing the transaction authorised to perform that transaction ?? ( example - spirits / alcohol sales).