Blue Skies Travel Charges Ok-Bong Lee a Total of $3,500

Blue Skies Travel charges Ok-Bong Lee a total of $3,500 for a trip to North Korea. After Lee pays the invoice, Blue Skies discovers that it failed to charge a special tax levied by the North Korean government on foreign travelers. Blue Skies sends an additional invoice to Lee, who refuses to pay, saying a deal is a deal. Which of the following is correct?

A) This is a mistake of fact on the part of Blue Skies. It is entitled to rectification of the contract - that is, to include the extra charge.
B) This is a mistake of law on the part of Blue Skies. The contract is binding on it.
C) This is a mutual mistake. The contract cannot be rescinded or changed.
D) This is a mistake as to interpretation of the contract. Blue Skies is entitled to damages.
E) Lee cannot take advantage of Blue Skies' unilateral mistake, and must pay the additional charge.