Use the Registered Mail option for more reliable delivery if you do not trust your postman or neighbours. This option doesn't come with the tracking option yet but it should be available soon according to the Czech post statement.
IMPORTANT: Since January 2011, we have noticed delays (1 month+, but 2-3 weeks on average and the situation should be getting better according to USPS site) regarding Priority Airmail delivery overseas (USA, Canada, Australia, South America).
Registered mail option is REALLY recommended for those regions until further notice.
Delivery within Europe has been really safe and fast (3-5 days) so far and there's no really need to use the Registered Mail option, especially for smaller packages, since the experience has been flawless so far.