How exactly does amazon trade in work?

I'm thinking about trading in my iphone to amazon I saw they'll give me $250 credit… And there's a item I'm looking to buy from them that's $255.So how would that work? Do I have to exactly that $250 or can I use the $250 and $5 off my credit card?

Title
Alliance Group Limited

descripation
Alliance Group Limited was established in 1948 by a group of farmers who formed the company to process and market
high quality meat and co-products to international markets. The company is a co-operative, wholly owned and
supplied by our farmers. It has become one of the largest exporters of meat products from New Zealand, and is
New Zealand's leading producer of lamb-based meat products.

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