I dunno Doomy. Not sure why it would occur to you to troll a clerk over the amount to prepay on your card to fill up. That doesn't seem like a worthwhile attempt at a troll unless you're flirting with clerk, and she's just going to give you the stink eye if you try it.
Also, not sure why everyone is going on about the size of the tank. It's not the size of the tank that matters, it's the amount that you need to fill it which should never ever be the tank's total size. I would assume you can guess at how much you need based on what the gauge says and what you've needed in the past to fill up from that point.