Hello please see the following ocnfigurations according to the requirements

We offer services (travel packages) that are booked with a downpayment and the payments are as follow:

- 30% when the booking is made (due date 7 days from date quotation issued) 

- remaining balance due date: 60 days before the service date

For the second part i would suggest putting the percentage on a long date as we may not know when we will pay the remaining balance, by putting along date we can also make make the payment before the deadline, payment terms are usually added as a form of control and agreement with the customer when to pay, if your balance is variable is better to put a long day rather than a short one 

This solution may be the only out of the box we have available