How do you handle donations bought with coupons for tax purposes?

I will be itemizing my deductions this year and am so confused! If I donated $500 worth of stuff but paid somewhere between $50-200 for it depending on how you count the deals, what do I put? What do I show if I get audited?

For example: I bought two toothpastes with a shelf price of $6, on sale for $4 each and then got buy one get one, and then I kept one and donated the other. Is it a $6 donation since that was the shelf price, a $4 donation since that was the sale price, a $2 donation which is the average price per item paid, or a $0 donation since I got the second item “free”?

Would love to hear your thoughts and would especially like to hear from anyone who has done this before! Thank you!