The power of attorney for invoices needs to be written, the name or name of the principal and the trustee, the residence and other basic information; the basic information of the invoice; the matters entrusted to the commission; the authority and scope of the commission; as well as the responsibility for breach of contract and other important content. According to the "Chinese people's *** and national code" article 143 has the following conditions of civil legal acts are valid: (a) the actor has the corresponding civil capacity; (b) the expression of meaning is true; (c) does not violate the mandatory provisions of the laws, administrative regulations, and does not violate public order and morals. Article 470 The contents of a contract shall be agreed by the parties, and generally include the following provisions: (1) the names and domiciles of the parties; (2) the subject matter; (3) the quantity; (4) the quality; (5) the price or remuneration; (6) the period, place and manner of fulfillment; (7) the liability for breach of contract; and (8) the method of dispute settlement. The parties may refer to the model texts of various types of contracts to conclude a contract. Article 490 Where the parties conclude a contract in the form of a contract book, the contract shall come into existence when it is signed, sealed or fingerprinted by all the parties. Before the signature, seal or fingerprints, one party has performed the main obligations, the other party accepts, the contract is established.