European (and Canadian and, I think., U.S.) air carriers all have the same restrictions on liquids:
1. You can only bring on liquids and gels in containers of no more than 100 millilitres.
2. These containers must be carried in a transparent plastic bag.
3. There is a limit of one transparent plastic bag per person.
4. The volume of the transparent plastic bag may not be greater than 1 litre [this means 8in x 8in, 20cm x 20cm]
5. The transparent plastic bag must be re-sealable.
Although I haven't tested this, I suspect you might get two boxed bell jars into such a bag, and maybe another long, thin box on top of that. I'm not sure why you're panicking, though - Guerlain and Montale should ship to Spain, and Serge Lutens does for sure, although it will be more expensive than buying it yourself. The only one I know which is absolutely impossible to buy outside of Paris (although I'm sure there are others) is Le Labo Vanille 44, which is only sold at Colette.
If you already have a perfume that you love which is hard to find outside Paris, get that one. Otherwise, I might suggest using the opportunity to try a few new things that would be very hard to find outside of Paris, and then if you love one, get it. If you don't find anything new that you love, you can still get something old that you love.
Bois d'Armenie isn't sold exclusively in Paris - they had it at the Toronto boutique, when I was there in February.