AutoZone Core charge on battery exchange North Vernon Nationwide

I purchased a 60 month "Duralast Gold" battery that quit after 35 months. Took it to the local store where I purchased it and they replaced it giving me a credit for the remaining months - $62.99. The problem arose when the total price was nearly $90.00 ($87.74). When I reviewed the charges with the clerk I found that I was being charged a core charge of $18.00 + tax. When I argued about the core charge, pointing out that the old battery was sitting right there, the clerk stated that he had used that battery to give me credit for the remaining portion of the warranty, so he could not then use it for the core and that was the way it works.

What? I am completely unable to fathom the reasoning here. Bottom line is I was charged an addition $19.26 or  over a 20% surcharge, 1/3 of remaining the warranty, for exchanging the defective battery. That is a total ripoff.