What is the correct site to book Eurail passes on?

Eurail.com is the official site, but each country has a subsidiary which sells these passes eg RailEurope, Railplus. But, if the trips you mention are the only train trips you intend to take, you'll find it much cheaper to buy point-to-point tickets rather than a pass.

based on todays prices

from Paris to Aix en P would cost around euro 110

and the secon travel 284 euro

Eurail is for non European residentials

the first trip is a one day trip

the second travel takes two days

for a 26 year or older person a 5 days ticket during a two month period would cost 303 euro

travelling by special trains thus faster trains needs also an supplement and not each train allows eurail users

example

let say you bought today a train ticket from Aix to Barcelona the ticket is a month valid thus at the 6 of august you can arrive at Barcelona