Don’t send ANYTHING until the money appears in your account – it can take up to 24 hours. It’s possible that your buyer has insufficient funds or a registered credit card on their account, which is why you’re seeing this message.
In all cases with eBay, don’t let anything go until the money’s in your account or pocket.
I’ve been using eBay for over ten years – there are a lot of to$$ers out there who will try to rip you off and scam you. My advice in a situation like that is to stay calm, be polite, retain ALL paperwork (including receipts and proof of posting) and as long as you know that you’re in the right STICK TO YOUR GUNS.
Be warned that eBay can be incredibly addictive and useful if used properly – I’ve had some real bargains and I’ve also made some good sales over the years, and I would say that my experience has probably been 95% positive. You’ll find that if you’re honest with people, they’ll be honest back. Always give a proper description of your item (if it’s valuable, e.g. a bike or a car, go into minute detail and be really long-winded – it puts off illiterate dickheads), be polite and have fun!