What is the difference between a postcard shipped via national post office vs a letter slipped into a DHL express envelope, in plain terms? Both serve the same

What is the difference between a postcard shipped via national post office vs a letter slipped into a DHL express envelope, in plain terms? Both serve the same purpose, transfer a piece of paper. One of them does not get delivered if you forgot to lick another piece of special paper onto it (stamps) before dropping it into the mailbox, oh and btw make sure you drop it in the correct mailbox, it is not the same mailbox where the orher envelope works. One gets delivered to the door while the other is delivered to your closest pickup point. Remote delivery is complex, even the old way. Sadly such technology long ago stopped being about making things simpler for customers, unless the entire end-to-end chain is inside their walled garden and making it ever harder to get outside of it to where all your other friends are.