Cashier did not listen to my billing requirements.

I purchased a cellphone on 02 October 2015. I distinctly told her twice that I needed the sale to go through on a six monthly basis. I even gave her the reason why.<br> Well, well, to my surprise, I get billed for the full amount for the end of October. Which, sets me back, interfering with my budget.<br> This is not the first time this happens. If cashiers will just listen to the customers' requirements before even processing the sale, it would be of great help.<br> They are just so excited or enthralled to make the sale, that they forget, I am the one paying for the item.