Apple is offering (https://support.apple.com/iphone-out-of-warranty-battery-replacement-credit) a $50 credit to customers who paid for an out-of-warranty battery replacement for iPhone 6 or later devices between Jan. 1, 2017, and Dec. 28, 2017. Eligible battery replacements were completed at an Apple authorized service location — Apple Store, Apple Repair Center, or an Apple Authorized Service Provider.
The credit will be provided as an electronic funds transfer or a credit on the credit card used to pay for the battery replacement service. Eligible customers will be contacted by Apple via email between May 23, 2018 and July 27, 2018 with instructions on how to obtain the credit.
If you haven’t received an email from Apple by Aug. 1, 2018, but believe you are eligible for a credit based on the terms above, contact Apple by Dec. 31, 2018. Note that proof of service at an Apple authorized service location may be required.