Postmarked by April 17th. If you have some reason to doubt the timely delivery, you should pay extra for proof of posting date. Further, write across the top "Taxpayer Abroad", and you're covered under the 15 June 2006 deadline for filing extensions. The 15 June deadline is for filing the extensions, not for paying the tax, so yes, you'll want to post your cheque 17 April - but you'll no longer have to worry that the extension "might not get there in time".