I have posted this before to describe how I accessed a foyer chandelier: I dragged the kitchen table into the foyer. Then I covered the table with a piece of cardboard. I placed my step ladder on the table, then placed a sturdy chair next to the table. I stood on the chair, stepped onto the table, then climbed the ladder.

Since you are dealing with a ledge, you could simple buy a $60 lightweight aluminium extention ladder and lean it against the ledge.

I have a two story foyer with a chandelier and window ledge. I bought the telescoping wand and several different dusting heads from Frontgate. They work well, but were expensive. Shortly after I bought it, I saw the same thing at Sear's Hardware for much less. I think if you look around you can find it for much less. I would check Bed, Bath and Beyond, Home Depot, Lowes and maybe online. Good Luck!

i'd probably go buy two sticks of pvc pipe
heat and bend one end to nice angle
and duct tape a duster on it.

Definitely don't feel comfortable standing on a ladder on the table. Will check the hardware stores. If anyone knows of a product offhand, I'd appreciate it!

Found a local janitorial supply company. They sold me a 12 foot extension rod and duster. Saved $50 off of Frontgate price.

They said it would reach 30 feet as you would hold the rod over your head (and depending on your height that would be 6 - 8 feet). Turns out that isn't quite true (for those of you who may be in the same boat) as the rod and duster, although the rod is hollow, is somewhat heavy to hold over your head and manipulate. I'm more comfortable having some of the rod below my hands.

Did work though. No more dust kitties on the ledge!

I found a 6' aluminum extension pole for about $10 at Home Depot; it extends to 12'. I taped a duster to the end of it at an angle, and I can reach 16' easily by holding it above my head. I wouldn't want to do that all day, but I'm good for 5 minutes.