How to handle manipulation of data after a db record is written from outside my program

Question

I have written a warehouse management web app. The application handles batch picking, warehouse routing, packing, and the final piece is handled by UPS worldship to "ship" the packages. Worldship will write a record to my postgres db after every shipment or void. I need a way to cleanly see that the record was written/deleted and then "do stuff".

The easy answer is to have a program just monitor the database and when it sees a record written to do its thing but something is nagging at me that there is probably a better way to go about this that doesn't have a program polling the table and comparing it to what was there last.

EDIT: How i was going to approach this... The table is setup with a status field. My program would on intervals read the table for any records that have a null status, if records are found "do stuff" then mark the status as complete.

TIA