Spanish ID card has multiple layered security in them. The obvious and difficult to commit fraud with is the chip which is just a cryptographic one, but you also have RFID in them (with I assume appropriate FNMT signatures), but also physically the patterns in the print, the different textures in different areas of it, holograms, transparencies and the like.
For most ID-requiring processes people undergo training to identify these security features, to the level of fraud that it's worth detecting for said process.
When the post office asks for your ID to retrieve a package, they won't check much, but I don't think it's unusual for banks to pass your card through the RFID reader and have a high res picture of your face on screen even if only to recognize you properly (btw you have apps to read such data).
For most ID-requiring processes people undergo training to identify these security features, to the level of fraud that it's worth detecting for said process.
When the post office asks for your ID to retrieve a package, they won't check much, but I don't think it's unusual for banks to pass your card through the RFID reader and have a high res picture of your face on screen even if only to recognize you properly (btw you have apps to read such data).