Put the zip code first

The PAF is about what the Royal Mail calls "Delivery Points" which are places they promise to deliver physical letters to. Inside my building for example every home has a front door, with a letter box, and letters literally get posted into my flat, but down the street there's a building with a rack of boxes set into the wall and post is delivered to your numbered box, and up the street there's a townhouse converted and all the mail just goes in one piles for everybody in that building.

Many delivery points share a postcode, but because they know them all they actually all have unique numbers, and unlike a postcode they're not for humans so they get changed quite frequently as new buildings are constructed or working patterns change, if you examine your post carefully (in the UK†) there are two rows of fluorescent orange dots on the outside, these were printed by mail sorting machines shortly after the mail was received, one is an entirely arbitrary serial number and it designates that piece of mail for a very short period (say a few days) to enable statistical tracking for performance - if #213940202 entered the system in Glasgow on Monday, was in Portsmouth by Tuesday but wasn't delivered to someone's door until Friday the problem ain't in Glasgow. But the other one we're interested in here is the Delivery Point as a numeric code. If you don't have multiple addresses this row of orange dots will be identical on every item you've received for some time yet it's different on someone else's mail.

† This trick was invented in Britain but is used (licensed) in some other countries because it's a good idea, however exactly what is encoded and so what it "means" may vary.

You CAN’T get the country from a zip code. France also use 5 digits zip code for example. You must ask for the country before asking for the zip code, even if you pre fill it with a sane default, like the main country you operate in.

For instance, my postcode covers 15 houses and half of a large park. Those houses have been subdivided into 4-6 flats, and many have been redivided or renumbered multiple times. The park also contains various buildings and other places that might plausibly receive deliveries, some of which have multiple entries (an electricity substation appears 8 times, for some reason). So in total, it covers more than 200 addresses - mine is no.140-ish in a typical sorted list.

You'd be amazed by how many address checkers can't handle more than 64, 100, or 128 addresses in a postcode. Or how many scrollbars stop working, requiring you to use the arrow keys to navigate. Or how many other glitches I've seen.

The other common problems are usually down to temporary postcodes (which used to always end with an 'X' but can now be indistinguishable from 'real' codes) escaping into the wild, re-addressings and re-numberings not being picked up properly, the old problem of outdated PAFs being used, and - my personal favourite - BT/Openreach using their own separate postcode database, dating from the Post Office split in the early 1980s, which doesn't always agree with the PAF. Agh!