A contract requires an offer and an acceptance. An offer is a manifestation of present contractual intent containing definite and certain terms communicated to the other party.Vendor's written quotation is a valid offer with definite and certain terms; price quoted is valid for sixty days and this was communicated to customer.  An offer is effective upon receipt.  The offer was effective when customer received it.Acceptance is effective upon dispatch.  Customer accepted all of the terms of the offer and communicated this to vendor by sending a written purchase order for the quoted price within the sixty day period that the quote was in effect.  Therefore, customer's acceptance of all the terms in the offer formed a valid contract.  Vendor cannot change the price.  Customer can sue vendor for breach of contract for issuing the invoice at a higher price.A contract requires a meeting of the minds (agreement on the essential terms which include identification of the parties, subject matter of the contract, price, quantity, time for performance, etc.) as occurred here.