Dear, there should be an invoice option for walking customer with customer name and contact number fields. Can you do that or is there already an option available in your application? And also please add cell number option in customer adding table.
the invoice is used taxable receipt of the product or services purchased by customer.
for walking customer u can use cash transaction receipt. @Hassan143
You can create a customer whether they are walk-in, internet, phone-in, or mail. You do not have to use the customer for a transaction, either now or in the future.
Use a custom field. See the Guide: Manager Guides.