CONSTANT BILLING AND ORDERS

I got a call from Mycom Trading 2015 regarding ink cartridges for my printer. I have placed an order, received it and got a fax from them with a less 2.5% discount if paid within 30 days of order 11/5/2015. Deal done and paid for. There after I kept on receiving phone calls to say my order is not received in full and I need to order more cartridges. This kept on insisting on these orders, so I agreed to receive another batch. Yet this was not the end!! On 18/08/2015 I received a call from Mandy with the same story. We both agreed that she will send me the last 3 black cartridges, as this was the most common one used. I asked her again for my letter stating that my said order has been completed. I have up to date not received this said and promised letter. Mycom trading keep on phoning me with now threating to sue me. I am tired of this and these people do not keep up to what they promise..