You're quite right. In fact, you got the whole picture about this complex postal code system.I'm not familiar with the Portuguese way of handling zip codes, but looks like most of Portuguese users don't use the TOWN suffix. Hmm..
Thanks for the answer and please forgive me for not having logged in previously (I just didn't realise the difference).
Thanks, obrigado, my fellow coutryman BogPoet. The Portugal Post site, http://www.ctt.pt is down now, but, if I find the time to do it, I'll be glad to help adding Portuguese post codes. This is not an official volunteering offer, yet
By the way, my nick is LM (from Luís Miguel), not IM. With the low case and non-serif fonts, I get this natural mistake all the time...

