I don't know what the prices of those places is but if you contact my boy Tito at Gotham Aquatics he's a vendor here he will hook you up and he delivers all over the place. Last I checked 200 gallon box of instant ocean was 45 bucks.
If the salt is hard as a rock then it most likely precipitated when it absorbed the moisture. A light hardening is fine but rock hard and need to use a screwdriver with force to break it up means every batch is going to be off with parameters.
Is there anyway to prevent the salt from hardening? I know by using it but I am just asking so maybe we all can take the necessary measures to prevent this from happening. I am sure it has happened to others as well.
Just keep it in a sealed container in a cool and dry place and make sure your not spilling water in it when your mixing your saltwater. Its that simple. It typically happens quick when you get old batches that have been sitting somewhere for a few years in a fish store.