Laurent,
the letter made a fast journey indeed. The mini album thing is called stock card, very handy item for temporary storage.
Allow me to give an advice: there are way too many stamps out there to collect them all, so maybe you should try and focus on one or more particular countries or stamp issuing entities which are of interest to you because of some personal relation/connection, or maybe there is a topic that you like such as football, farming, chocolate, frogs, etc. etc.
Have a look at
https://www.delcampe.net/en_GB/coll...bles/stamps/to see what countries/entities and what topics exist.
Some countries are "closed" meaning that they stopped issuing stamps for some reason, for instance Mayotte with 172 issues between 1892 and 2011.
For many topics some countries flood the market with issues, so try to choose one where there are not too many stamps out there. Also nice are definitives (frankeerzegels) because in general they are not expensive and in many cases they show typical themes of the respective country.
Countries I collect are Germany, Netherlands, and some of the older German entities like Saar, French Occupation. Some of my topics are windmills, the postman, one-stamp-per-country.
Enjoy.
Klaus