Unkept promises and fishing for interest free loans from customers

Amongst others they offer on-line Power Back-Up Systems of many shapes and sizes. I selected one 3000 VA Plug and Play system with 2 Lithium batteries. The specifications price were good and the delivery time of 1. or 2. week September was perfect for me. mot bus and after some back and forth they agreed to accept a 25 % deposit on order. Whem I received their official order document a new delivery date of October was stipulated. On my query they refused to amend their order document, claiming it was an "official document" and therefore not amendable. As I was verty dependent on the original delivery date (I explained my situation to them) and could not accept the new delivery date they cancelled my order. I have a problem of paying full Whack on order (not even laid eyes on the product) and accepting a newt delivery date (4 weeks later) without any written commitment from the company----only a verbal promise.