Again someone who has chosen the trains with the highest supplement to pay on a pass.
You cannot book online with a pass on Tr-It and certainly not for INTernational. have to do at a normal ticket counter. Also I think there is no direct train VNZ-Par, you have to change at least in Milano and thus make 2 bookings. Par-AMS direct is by Thalys. Especially all INTernational superfast trips in+out of FRance cost a lot more. You might be able to lower it by using slower trains and make more changes-sometimes a frightening thing for those who never used EUR railways.
In theory it is always possible that a train as nr 1 on the wish-list is fully booked (or the smaller quotum for passholders is taken, not all seats), but here in EUR there are nearly always plenty of alternatives.
On the site of eurail-read general info, REServations for more info.
If the supplmt on EUR* seems too high for you-plenty of much cheaper flights too, easyjet to 4 airports near LON, pay extra for large luggages.