Here’s an interesting mashal:
Imagine you go to a foreign city and find a food with a local hashgacha that you have never seen before. You call up your Rabbi back in NY and ask him if you can rely on that hashgacha. He says “yes, but only for certain types of foods”.
In this case, without ever seeing the actual product, your NY rav is basically giving his “hechsher” to the product that you are buying.
Same thing with the Domino sugar. You don’t know whether it adheres to your strict kashrus standards but Rabbi Heimish does. So Rabbi Heimish puts his own hashgacha on it to inform you that it does. This is a service that he offers, he charges for that service. Everybody needs a parnasa.