Please do not email purchase orders - see FAQ. Please order through this website and pay securely by card or Paypal, or select 'Pay Offline' at the Checkout stage to pay by invoice. There is a box to record any alternative invoicing address at the Basket stage and a field for purchase order reference at the Checkout stage. Payment by invoicing is only available to UK schools and for orders of £100 or more. You will receive an emailed invoice and payments can be made by card, BACS or cheque. Please do not select 'Pay Offline' for orders of under £100. Unfortunately we can no longer offer proforma invoicing as the administrative costs do not allow us to maintain the minimal pricing on our self-published products.