How to cancel a transaction? Need help ASAP.

I posted some items on Ebay Canada and when filling out the listing form I had the no international shipping box checked. When you went to the "shipping and payments" tab on the listing the only country that could be selected was Canada. Anyways, I had someone from the USA buy it (I know now it was because I didn't block international buyers in my settings) and it said to send them a payment invoice (On Ebay, overtop my "sold" listing). I decided I would and using there zip code I got an estimate, I took the quote and put it in the shipping area. I sent the invoice and it said awaiting payment but the item is deemed sold and no longer on Ebay. The person only had 3 feedback but I decided to give them the benefit of the doubt (I'm a new seller also, large on Amazon). Anyways, it has been close to 48 hours and still no payment but I noticed they had three more feedback. The last three feedback are literally stating "SCAMMER BEWARE" , "Scammer....Beware" and "Scammer". All from people with a # less than 20 next to the star (same as myself, currently 16). This obviously is just asking for trouble so I would appreciate if somebody could give me advice since I'm not 100% sure how Ebay operates.