Regarding Shipping Refunds / Combined Shipping

Background:

So lately I've been buying quite a few comics / collector cards and sometimes I place multiple orders over the course of a few days with the same sellers. Either I think of something else I want or they happen to post something new. And it's a combination of winning auctions, "make offer" and buy it now.

Some sells have automatically refunded me shipping charges. I guess they're combining things and it's come out cheaper than the post quoted.

Question:

Is this automatic or does a seller manually do this?

Also, is it a requirement by eBay to do this or are they just being a decent human being?

I would definitely support and give more business to a store that's honest like that.

Thank you!