Oops, Dad died earlier this year, and guess what did with his things.
Make a very careful list of cost and condition and current value...If you did not get photos go back and find them from family gatherings etc. Receipts from the agency are usually blank...fill them out...12 boxes, 32 items on hangers...etc.
Submit it with the return...they do not need it, but will have it and know you did your homework...haven’t bothered me about donations with this system and I have used it for years.
Next time take photos of the items...piles or boxes and large items too.
I’ve since learned that large charitable donations such as these always get the 5th degree, so it’s best to get your receipts in order, and if you don’t have any, take a smaller amount on a deduction than you actually gave away...it’s just not worth the audit.