Some Ebay buyers just suck

Hi there! Well I have one issue wih ebay that I just wanted to discuss. Actually it's not necessarily an issue just yet, but it could be.

So I was selling something on ebay for roughly 3 days, and I got a bid by 1 person, and they ended up winning. Yay! Sold!!!! It was just a bottle of perfume that I owned, but never used it, and it was still brand new in the box. The auction total for this buyer is $18.50. Hey, I'll take it.

Well once the auction ended, I got the email from ebay that the auction was over, along with the buyer. I decided for the heck of it to send the buyer an invoice telling them how much they owe, just because.
I have not recieved a payment yet, and it is has been 4 days. I decided to send the winning bidder an email today, just reminding her of the payment due. I have not heard baack yet, but I am wondering if she is going to blow me off.
The reason why I say this is because I had a winning bidder one time who never paid for the item he/she won, and never responded to the 2 or 3 emails I sent. Apparently I was not the only one who had issues with that person, and because of that, that person's feedback rating was -3 I believe. Yeah, not good!

I just hope this is not the same situation as last time. I need this money, even though it is not even $20.00. Got to fill up the gas tank, ya know?!?! Why can't people just pay?!?! If you can't afford it, you shouldn't be bidding on it.
I think the next time I put up an auction, I am going to add...serious bidders only, payment must be received no later than 72 hours after the auction, etc...
I will wait a few more days, but if I don't see a payment, I am sending another email and if I don't see anything after that, I am going to re-list my auction, and I am leaving her negative feedback. Oh yeah, I forgot to mention, this person only has a feedback rating of 2, so that could be trouble. A newbie!!! We will see...