Very useful information!!
For Spain it is very easy:
MAIL TO SPAIN
The typical address is
Penpal Name and Surname
Street Name 123, 1ºA
12345 City, Province
Country
1. Here, you know, we have two surnames, so the name in the letter could be quite long. It is not mandatory, mailmen just deliver the mail to the address unless it is some sort of official mail, which usually has to be delivered personally and you have to sign that you receive it.
2. The name of the streets depends on its category and it has an abreviation characteres before the street name:
c/ - Than means "Calle" and it is the usual street.
Avda. - That means "Avenida" and it refers to a very wide and important street of a city.
Pza. - That means "Plaza", a square.
There are more categories, but they are not common and usually full written. You can have "Paseo", "Vía", "Rotonda", etc. The street number is written after the street name.
3. In cities here there are few unifamiliar houses, mostly big buildings of apartments. So, normally, you have some numbers and letters after the street name and the street number. You will have at leats one number which means the floor, and letters which depends on the appartments on that floor. In my building there are 4 apartments in each floor, and they are called A, B, C and D. So my appartment is 5ºA. (For the exchanges I just write my address for you as 5A to avoid symbols and that works too, but the real correct should be 5ºA). At other buildings you can see just two appartmens called "dcha" (abreviation for "derecha", or right) and "izda" or "izq" (abreviation for "izquierda", or left). Sometimes I have seen 4 appartments in each floor and called "dcha-izda", "dcha-dcha", "izda-izda" and "izda-dcha". It can get complicated, but if you just write those numbers and letters as they are given to you, next to the strret name and number, you will be fine!
4. The zip code is always before the city name. And we also write the Province name, after the city.
MAIL FROM SPAIN
The spanish mail service is Correos. You can take your letter to an office and pay there for postage, they weight your letter and tell you how much does it cost for where it is addressed. They have a nice website, where you can estimate the postage costs at home:
http://www.correos.es/ss/Satellite/site/pagina-calculador_tarifas/sidioma=es_ES It is possible even to pay that postage from home (via credit card or paypal I think) and then print a sticker with their codes and just give it as it is at the office. (But, that sticker is not calligraphied! ). If you pay at the office they put a smaller (but quite big) ugly sticker with some codes. At some offices you can ask for stamps, but not always.
Stamps are only sold at tobacco shops (called Estancos), but nowadays they only carry the strictly neccessary because people don't buy a lot, and it is not usually the international stamp, although you can add as many stamps as you want to get the neccesary postage. People here only sends letters for wedding invites, and only for those who can't give in person! So, if you want stamps you need to contact Correos, where they sell full sheets of decorated stamps for collectors (but usable! and you can choose from the full catalog).
About stamps, there is also a nice service Correos offer. Usually for weddings, you can order personalized stamps (usable!). They will use your images to make full sheets of current stamps, with some conditions (I think you can not use real-look human faces, and things like that). Those stamps will have the normal postage, but they are going to cost you quite more because of the personalization service.
If you have stamps, or pay in advanced via the website, for your letter, it is not neccesary to go to an office to send it. There are, at the streets, big yellow mail boxes, there you can insert your letters (there is no way you can get them back once inserted) and the mail man will take them to be sent.
FLOURISHES: I have absolutely no idea. I know that normalized envelopes and addresses cost a bit less. Those are the ones which are automatically manipulated. For that it is neccesary to be a rectangular white envelope (they accept other light colors, but not blue) with some limits on the measurements , written with black or dark ink in comercial fonts (no flourishes, no caligraphy, no lettering, no even underlined letters!) and no more that 20 g. If the letter does not fit, it costs more, but it is sent. I only know that they do not sent anything smaller than 14 x 9 cm, or anything with draws or texts violating any human fundamental right.