The Salvation Army, or any other organization that accepts item donations, will not assign a value to the donation. It's up to you to determine the value and support your valuation. Keep an itemized list of what you donate, pictures, etc., and documentation of how you determined the value.
See IRS Publication 526 for information about the records you must keep.
Read online: https://www.irs.gov/publications/p526#en_US_2017_publink1000229844
Download PDF: https://www.irs.gov/pub/irs-pdf/p526.pdf
