For a limited time Apple is offering up to US$100 of additional trade in credit when you trade in certain iPhones. This can be used towards an iPhone XS or iPhone XR.
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Trade In Credit
Models available for trade in credit include: iPhone 6/6+, iPhone 6s/6s+, iPhone 7/7+, iPhone 8 (but apparently not iPhone 8 Plus). In the regular trade in process Apple offers US$150. For more information visit the Apple GiveBack page.