I've had two cases of excess over order supplies John, but both retailers told me to keep the extra. Im surprised your battery company didn't do the same since the cost of all 24 to them was probably only about £20. Since they'd already had £4 from you and it would cost several pounds to return the surplus, it really wasn't necessary to pursue it further.
Most companies making a mistake are far more generous, the most notable example I've known being Giant Bicycles.
They sent a dealer a replacement £500 Panasonic crank unit for the Lafree, something he'd never ordered since he wasn't their e-bike agent, just an agent for their unpowered bikes. He mailed their Netherlands centre to ask if they'd prefer him to return it to them or send it on to whoever had ordered it.
Their response was to tell him not to go to the trouble of returning it, suggesting he might like to sell it and donate the proceeds to the charity of his choice. He put it on ebay and I bought it, for £16.
On another occasion when I received a double order, the value of the extra item being about the same as your oversupply, the retailer, a small company, replied:
Thank you for informing us of this. Since it was our mistake please accept the addition with our compliments.
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