Juste got the feedback from a customer.
Paypal is adding a 20% tax on the product.
My product price on the website is with taxes included (actually, I put the final price for the product price, and added a 0% taxe on it ... Which should be like having product price without taxes + taxe ... it's easier for me this way)
Anyway, here it is in summary :
In website cart :
Product price (with 0% tax) : 35,90 €
Shipping : 6,45 €
Total displaying in the cart : 42,35 €
BUT
When I get to Paypal :
Product price : €42,35
+ TVA (20,000 % en France) : €8,47 (where does this come from ???)
Total : €50,82
I tried to turn off "send order details", but it's the same !!