Did you say in your listing that you can't post every day? If you didn't then it is being aggressive to remind people to pay the very next day. I'd get aggro too and deliberately delay payment, though I wouldn't use the language because I don't swear.
What I would have done after the first reply was apologise and explain. All you did was exacerbate the problem by threatening a non-payer case.
If you want to be paid straight away then tick the box requiring immediate payment, or tell people what days you can post and then extend your handling time to accommodate it regardless of what day of the week they pay.
In my opinion you brought this on yourself by harrassing someone for payment in an unrealistic time. You might have got a better response (at least a more polite one) if you'd sent a polite message the next day and explained your situation, and given them a chance to explain theirs. They weren't breaking any rules by expecting to pay a few days after the sale.
