You have to pay fees on items you sell, and if you have sold many, that may be what the invoice is for. It will say on the invoice, - is it those final value fees?
As for your deadbeat, if it has been 4 days and no payment, time to open an Unpaid Item dispute:
http://pages.ebay.ca/help/sell/unpaid-items.html
If still no word or no payment from the buyer after another 4 days, you close it, get your fees back, and the buyer gets a strike. Then you add the person to your list of blocked bidders here:
http://pages.ebay.ca/help/sell/manage_bidders_ov.html#block
And to keep away habitual nonpayers, you can block all those with 2 or more strikes in the last year here:
http://pages.ebay.ca/help/sell/manage_bidders_ov.html#set
Some folks here will tell you to block anyone without a paypal account but they don't do it themselves because they don't want to miss all those credit card transactions. But if you are beleaguered with nonpaying zero-feedback newbies you might need to resort to it for a time, anyway.
So a couple links about fees, - this rather exhaustive one about selling fees:
http://pages.ebay.ca/help/sell/fees.html
And those final value fees in particular:
http://pages.ebay.ca/help/sell/fvf.html
