I'd maybe send another invoice today with a note asking them to let you know when they are going to pay so that you can get the item posted out to them, nicey, nicey.
Then after another couple of days (maybe Weds) I'd send another asking for contact or payment by a specified date (say next Monday) or that you will have no choice but to raise a non paying bidder thingamy but still saying it in a nice way IYSWIM.
Don't panic yet though, some people are just really slow and will usually pay up when they get the second invoice
There was a thread about hassling buyers for payment just the other week and the OP got very huffy when the majority of people felt she had been OTT and a bit aggressive! The buyer has 7 days to pay, after that you can open an unpaid item dispute, they have a further 8 days to pay and if they still haven't paid you can close it and you get your listing fees back. After 4 or 5 days I send a quick message saying your item is packed and ready to be posted I'm just waiting for your payment. If I receive no reply to that on day 7 I open the dispute. This often spurs them into action although not always. Although you and I pay promptly it seems some people bid on an item forget all about it and don't turn on their computer for another week!
the 7 days are 7 calendar days as you can pay via paypal 24 hours a day, 7 days a week but you can't open the dispute til the full 7 days ie if the auction ended at 8pm then you can start it 8pm onwards - 7 days later. Same with the dispute it is exactly 8 days later before you can close it. GL
What a disappointing first selling experience after the initial excitement! Most buyers are OK but there are a few who are nuisances - try not to let it put you off, and just calmly go through the procedures as described by Smac. Does the buyer have low or poor feedback? Are they UK based?