TOTAL OF BILLS, PAYMENT DETAILS

1. opening and closing amounts are automatically computed
2. Payment details are shown depending on radio button clicked.
*BILL: if one input is invalid, total will not be computed ? or just
omit the invalid input and compute the rest?
*PURCHASE: napansin ko lang, if kunwari meron na sa purchase list ng
exisitng item, tpos ng purchase ulit nun, ni-ooverwrite siya? ganun ba
talaga or dapt malilista ulit sya?