I'm a little confused. You returned the product and it was refused at the post office? Do you mean you TRIED to return the product but the post office wouldn't allow you to ship it? If that's the case, then you have two problems: 1) The company never received their product so they won't refund any money; 2) you cannot ship a container that could leak without it being secured somehow (which, based on the product type, is what I assume happened).
Your credit card company will not automatically remove charges. They have to determine if you are due a credit first. It's not immediate.
Also, I'm not sure what going over the limit has to do with this. You shouldn't be charging anything to your card if it's maxed out.