a)When you select a customer, number and email of customer have been show .
b)With create each invoice,invoice number incease automaticly.
c)Invoice date is date of system & billing due date is three days after that.
d)When you select a service name,information of service for example:Service Number,Charge,Explanation;have been show.
e)Discount typed with % or in digit format.
f)Each number that type in 'other Charge' fields add with number of 'charge' fields.