When going through the submission process you'll see it prompts you whether you want to use your standard (free) credits or premium credits. If you don't have any premium ones -- and don't want to buy them -- make sure you stick to the 'standard' path.
The core idea behind SubmitHub's "model" is that for decades it's been near-impossible to catch the attention of bloggers/playlisters/curators/whatever. Our system dangles a carrot (~$1) in front of them to guarantee a response+feedback about your song (with the ideal outcome of course being that they share your song - no additional costs involved).
For pro musicians who are committed to your product as a promotion path, perhaps they don't care about that credits ambiguity because they go in with a marketing budget and know what they're going in for. For me, I will stick with making music for fun. I wish your team success at their mission.
There is an interview with the founder on a podcast somewhere that's quite informative. I'll link to it if I can find it.
I had never heard of Soundcampaign though. Will have to check it out.
If you want to try submithub out, be ready to pay with a card, regardless of what the UX seems to imply. It seemed like a good service, but now I don't see myself dealing with them.