Hi Rica,
This is a late reply but yes it usually takes more than 7 days to reach UK and most countries if you use International Postal of Xend. So far two weeks is the norm but it can even take as long as 6 weeks to countries like Russia.
Unfortunately, Xend nor any courier has no control of the destination country's customs clearance so it's best that you ask your buyer if there are any restrictions on their end. :)