@K-L
Ok, thanks to Davebraco I could finally download my purchase.
1° Select your product
2° Click on buy
3° Since you haven't got the money, you will be prompted to put the software in the Shopping Bag
4° Go in the shopping bag
5° Click on checkout
6° You browser opens on Paypal and you can pay (with VAT and so on)
7° One the payement is done, close Amistore
8° Launch Amistore and your balance will have the amount you've just paid
9° Click on Shopping Bag
10° Select the product you've just paid
11° Click again on Checkout
12° When prompted, select Use Balance
13° You program will be downloaded
I hope it will become simplier in the future