While I understand where you're coming from... and have personally found the customer service pretty poor at Bunnings, I'm not surprised they didn't spend too long looking for it.

The pack is small enough for someone to slip into their pocket and walk out of the store, which probably happens more than they like to admit, so their stock numbers aren't going to be accurate for this sort of item. At the end of the day, I wouldn't be putting blame onto the person that served you, the issue was caused by who ever stocked the shelves (or moved the product).

This is coming from someone who worked in retail for a number of years.

Although, as I said... Bunnings has never been very good on the customer service front. I'm surprised you were able to find someone to help you at all. ;)