What you do now is get a lawyer. Do not say one more word on the subject. Not in writing, not in public, not even to your friends over beers, until you get a lawyer. Think of it like the Miranda rule -- now that a legal dispute has arisen, anything you say can and will be used by them against you. What you shouldn't do now is panic. They've sent you a letter demanding a large sum of money? Big deal, anyone can send a…
Bad news is, I did drop them an email saying that I hope to solve this amicably. I suppose I should still go get a lawyer right?
When you've found a lawyer, he/she will advise you on what to say and not say from that point forward. In the meantime, the safest course is to say nothing (or at least the absolute minimum you can) from now until then. If you've already sent an email you can't take it back, obviously, so when you get a lawyer be sure to give him/her a copy of the email (and any response you receive) so they can advise you on how it affects your position.
Good luck!