Hi, i'm not really looking for a huge solution for Ecommerce,
but more something simple to add the possibility to subscribe at a magazine
(send by post)

I'm looking for something that take credit card

something simple like that :
https://secure.readersdigest.ca/eng/mag_sub.php?partner=mag_cap

Thanks

Seacrh http://www.hotscripts.com. May be there are some scripts (even free) which support what you need.

I believe it's still in beta release but I've been hearing a lot of buzz about Magento. It should be a great product once it's in production.