1)I am in process with CapitolOne. My experience is this: Make sure you want to go all the way. If they fight it, the CA is a huge process, very time consuming, which may lead to a visit in the Tribunal (as I understand it.)
2) I've seen several people post success sending only the endorsed coupon to the regular payment address, and been credited. See Jed above. I sent to the "special payments" per the course instructions and they've cut off the cards, closed the account and claiming I still owe all the $. Total resistance. For more info on my experience see my forum post "Continuing Process with Capitol One."
Good luck!