Manuel Reservations should be able to determine payment procedures

Hello, I want to be able to set payment terms when invoicing guests. Common practice on other platforms allow guests to pay 50% at time of booking and 50% 30 days prior to check in. When I manually enter a direct reservation, the invoice sent requires 100% payment and security deposit. I would also like to be able to add a feee to a guest reservation after the invoice was sent. At times, guests request additional guests or a pet that require a fee.