Why did I get aprovel for $800 on my iphone and now Im getting charged

I traded in my phone, Mine was in mint condition they approved me for $800 value but now they are charging me full price for a new one? WTF, they also threw in a google watch and said it was free but then added a line for it so Im paying $29 for the line and charged me full price for the watch? WTF so my first bill was over $400, and now Im paying $200 that's $100 more than I paid with CC. They have the stores scaming people left and right and lying to customers to get commission. Do not switch to T mobile