Delivery wants more and more top up.

I wanted a couch, but couldn't order online due to my area being out of their delivery zone, but a very nice sales lady assisted me to do everything( which is why I am giving 2 stars!. ) The problem started when I asked them for a delivery quote, i knew it will be higher than the normal price, and they did charge me extra. 3 days later, they called and said my delivery amount is short, (charging me extra again) they can't deliver unless I l top up, yet they are the one who gave me the first amount which was fully paid… like why am I adding money, yet you already charged me..