eltiseba,
I have been modeling the same thing for quite some time now. This is the way that I have been doing it:
1. Model the bottle in standard, and shell the bottle, but do not remove the top surface, let it there. I do this because I am assuming that the cap is infinetly strong enough to hold the pressure load
2. In mechanica, add a constraint to the top surface and hold it fixed. Goes back to my assumption that the cap is infinetly strong.
3. Add a pressure load to all the internal surfaces of the bottle and then run the analysis.
The only problem with this method is that I am not taking into account the variable thickness of the bottle, but when I have been setting up my models, I have been shelling them to the min wall thickness. This is fairly simple method, but I have been getting pretty good results as far as deflection and stress compared to the actual parts.