Your credit is spent whether or not they share the song, but they are required to give you feedback if your song is rejected. Typically it will cost 1 credit per submission, with a few blogs/curators that charge up to 2 or 3 credits because they are of higher quality (this can mean a variety of different things but typically it's because their reach/audience is higher or they provide better feedback etc). You can then use these credits to submit your song to blogs/playlist curators/labels to listen and they will decide whether to share it or not. There are different packages but basically the more you buy the more discount you get. You buy premium credits which cost roughly $1 per credit. It should also be noted that I only aimed to get my songs on playlists and didn't care much for blog features or feedback. I am offering my opinion which may differ from others based on their experiences.

This is just one more to add to those for reference, in hopes that it will help you guys make a more informed decision whether to use these services and how to approach using them. I have read a lot of different views and cases from different people on this site with both good and bad experiences. I am only speaking from my own experience. Note: This is a long read so if you're not interested in my experience you can scroll down to the 'Conclusion' section where I give my opinion.

Sites like playlist push