Purchase Function

-able to search item by entering bar code
-prompt if the barcode is invalid
-prompt if the fields are empty
-'ENTER' button will add the item in the cart
-dropdown list customers (not yet dynamic, should drop existing
customers)
-'PURCHASE' button transfer the cart details to tables 'TRANSACTIONS'
and 'TRANS_DETAILS'
-after purchase, will be redirected to the same page with empty cart