Assuming that you want to use a script...
Go To layout [Invoices]
Enter find mode  //clear pause check box
Set field [Invoices::Shipment ; "Shipped" ]
Set FIeld[Customer::Status ; "Active" ]
Set Error capture [on]
Perform Find 
This finds all the records a summary field on this layout would then report the total value of these invoices. The total count of records in your found set will tell the number of invoices. (Not sure what you meant by "total amount of invoices".
This can also be done without scripting.
Define a new relationship:
AnyCustomer::anyfield x Invoices::anyfield
(AnyCustomer is a new table occurrence of Customer created by selecting Customer and then clicking the button with two green plus signs.)
Define a summary field in Invoices that either computes the total invoices ( or counts them).
Place a portal to Invoices on a layout based on AnyCustomer. Specify this portal filter expression:
Invoices::Shipment = "Shipped" and Customer::Status = "Active"
make this a one row portal and place your summary field in this portal to display the total.
(This example minimizes the number of new table occurrences needed, but requires a new layout just for this total. It's also possible to use different table occurrences in relationships so that you can place such a filtered portal on either a Customer or Invoice layout.)