You may customize the bill depending on the amount, number of payments, custom field and quantity field. This can be adjusted when you open the bill. 

By default, a new bill asks for a fixed amount. Unselect the Fix Amount option to collect open amount payment.

A new bill is set to receive multiple payments. Unselect Multi Payment option if you wish to only collect the payment once.

For each new bill, the default fields such as the name, email address and contact number are required. You may add other custom fields and enable the quantity field according to your usage.

You can also include a description of the bill adding an image or link for reference.