You broke a golden rule of FlyerTalk - never, EVER, contact the hotel/airline/whomever about a deal that you know you got but was "too good to be true".
You're new here, so I won't berate you. 'Deals' are obtained through mistakes ("bank error in your favor" as the Monopoly saying goes). Contacting hotels/airlines prior to flights/stays makes them aware of the mistakes. They, in turn, prevents deals from being bookable anymore and/or cancel existing deals (when allowed).

As to whether Agoda is responsible here probably depends on their terms & conditions. In general if a credit card is charged a "contract" is complete and the person who charged the card is 100% liable for what goods/services were offered.