Mercury is heavy, that's for sure. But a small quantity is not going to break the bottom out of a glass bottle. Now, if you have a ten gallon jug of water, and you try to carry it by the neck, it may snap clean off -- making neck weakens the glass. But a few mls of mercury is not going to strain a decent bottle. You never really know when you've gotten a crappy bottle though.
I gotta parrot macman104: 'tho. You're not out to establish a basement Superfund site. You want a ml, or 10 or 50, but not hundreds or thousands, so what does 50-100 ml do for you, that one or two doesn't? It's a metal. It's liquid at room temp. It's in a vial. How much do you need, just to look at? How durable a vessel do you need, just to set it on a shelf?