The shampoo problem is very easy to fix. Just squeeze out most of the air, then close the bottle before packing it. It should then have a "crumpled" appearance. The smaller amount of air inside will expand at the lower pressure in the hold (or in the cabin) but not enough to pop the cork. I always take this precaution with lotion and so forth. As an extra layer of safety, tape the cork closed. Since I started to do this, I have never had a problem with goo all over.
Of course, if your shampoo bottle is full, you have a problem.
As for pressurization, the hold IS
! Think about the shape of the airplane. It's easier to make the hull airtight (it's round and already strong) that to make the flat floor a pressure bulkhead.
"There are no stupid questions, but there are a lot of inquisitive idiots." - John Ringo