It sounds like you were the victim of a hard sell, given that you want to cancel the contract the very next day.
The three day right of recission applies to Retail Installment Sales Contracts, which involve payment over the course of more than three months and finance charges. I don't see it here.
However, the Mass. Attorney General has regulations which I believe will apply. I have copies and pasted the applicable one below for you to site in any correspondence.
If you want to cancel the agreement, send them a letter telling them that you believe you were the victim of Unfair and Deceptive Practices in that the company misrepresented the terms of product they were selling so that you were coerced into signing an agreement for more money that you intended; based upon their misrepresentation. Cite the regulation below, and indicate by a cc. at the bottom that you are sending a copy of your letter to the Attorney Generals Office. And, depending upon how strongly your feel about this situation, consider sending a copy of the letter to the Attorney General's Consumer Protection Office. They are very helpful- consider calling them for advice as well.
Your claim sounds fairly strong, the only downfall would be if the situation is interpreted as your having changed you mind; not having been tricked.
In any case, most reputable companies will rescind a contract in these circumstances. They don't want to have to chase the money; they don't want to have the attorney general involved; and they want a good reputation. .
Hope this information helps.
ATTORNEY GENERAL'S REGUALTION (CMR-CODE OF MASS. REGULATIONS)
940 CMR 3.16: General
Without limiting the scope of any other rule, regulation or
statute, an act or practice is a violation of M.G.L.c.
93A, § 2 if:
(1) It is oppressive or otherwise unconscionable in any
(2) Any person or other legal entity subject to this act
fails to disclose to a buyer or prospective buyer any fact, the
disclosure of which may have influenced the buyer or prospective
buyer not to enter into the transaction; or
(3) It fails to comply with existing statutes, rules,
regulations or laws, meant for the protection of the public's
health, safety, or welfare promulgated by the Commonwealth or any
political subdivision thereof intended to provide the consumers
of this Commonwealth protection; or
(4) It violates the Federal Trade Commission Act, the
Federal Consumer Credit Protection Act or other Federal consumer
protection statutes within the purview of M.G.L.c. 93A, § 2.