The up-front cost per unit is the most obvious number, but it's not the only one. You also need to factor in shipping, which is often based on weight and size, and the cost of storage. Batteries have a shelf life, and if they sit in your warehouse too long, they might lose capacity or even become unstable. That's a hidden cost that can eat into your margins.

Then there's the cost of returns. If a customer gets a defective unit, you might have to pay for return shipping, inspect the device, and possibly refund the full price. Multiply that by a few percent of your sales, and you'll see that it adds up. A more reliable battery might cost $2 more per unit, but if it cuts your return rate from 5% to 1%, you're saving money in the long run.

Finally, consider the cost of brand damage. A single safety incident can generate bad press and erode trust in your store. You might have to pull a whole product line, which means lost inventory and lost sales. That's not something you can easily put a number on, but it's a real risk.