1. Some bicycle pumps are of a design that permits the head to unscrew from the cylinder, causing them to leak air from the tube when you try to pump it up. Do not confuse this with a failure of the puncture patch in the tube, for you will get all oily changing the rear tube unnecessarily. But if you do change the tube, do make sure the puncture patch in the other tube you've been holding as a spare really has held, or you will find you are unable to pump that up at all, even after you've worked out why the pump isn't working either: then you will have to change the tube back again.
On the other hand, you will be able to reduce the time and sweary effort spent changing a tube to something more respectably efficient than hitherto.
2. Your seething discontent at the mysteriously complicated business that detains people ahead of you at the Post Office counter can be countered by the realisation that a mis-copied postcode on your parcel can cause the counter staff some tine-consuming detective work.