11 The plaintiff's claim is for $270,953.56 as being due under an agreement made with the defendant, on or about 12 August 2005, for the supply to the defendant of 5,391 square metres of concrete precast panels for $593,101. The agreement is alleged to be constituted by a letter from the plaintiff dated 17 May 2005 and a purchase order from the defendant dated 12 August 2005. The plaintiff alleges that it was a written term of the agreement contained in its letter of 17 May that any precast panels supplied to the defendant additional to the 5,391 square metres be paid for by the defendant at the rate of $110 per square metre plus GST. In fact, it is alleged, the plaintiff supplied additional panels to the extent of 914.67 square metres in respect of which it allowed the defendant a credit of $57,066.50 plus GST in respect of certain works undertaken by another supplier for the defendant. Overall the defendant became liable to pay the plaintiff $636,648.21 as to which the defendant had paid $353,420.04. Allowing some credits totalling $36,906.75 the defendant remained liable for $246,321.42 which, with the addition of GST, produced the amount claimed of $270,953.56.