The letter they sent was just an offer, they will likely negotiate with you, don't be scared to call them. Think about this stuff first:
(1) Do they have a judgment against you? If yes, you have lost some negotiating power, but they will still work with you...
(2) What can you afford and why? Call them and offer it, be specific with how you arrived at that number. If someone calls me and has thought about their other bills and offers me something they put time into calculating, I take them so much more seriously. We hear "I can't afford it" more times than you blink everyday, trust me, think of something more factual to offer them.
(3) Do you have any medical of financial hardships? (other civil claims, health issues, etc?) Tell them all of those things.
(4) Make sure you continue to tell them your willingness to settle. Anyone with sense will work with you.
Questions? Ask me.
You may have the ability to negotiate, but since your received the judgment, you are really behind the eight ball. If they are unwilling to negotiate and are going to garnish, you can look at bankruptcy. Depending on your situation, it may save you money in the long run and not be as bad on your credit as anticipated.