TL;DR - No, it is not payola and it does not violate Spotify's terms.
Definitions of Payola:
A good starting point here is to look at the definition of payola: "undercover or indirect payment (as to a disc jockey) for a commercial favor (as for promoting a particular recording)." That's a bit antiquated though, and you'll rarely deal with radio stations on SubmitHub.
In all likelihood, you're probably asking this question in the context of Spotify. Bullet #7 of their User Guidelines says the following is not permitted: "selling a user account or playlist, or otherwise accepting or offering to accept any compensation, financial or otherwise, to influence the name of an account or playlist or the content included on an account or playlist."
What you're paying for on SubmitHub:
If you choose to use premium credits, you are paying for each curator's time spent listening to and considering your song. To earn your payment, they must accomplish the following:
If they do not accomplish the above, you get your money back.
Curators do not earn extra money for sharing your song. They are paid the same regardless of whether they agree to share it or not. You cannot pay them extra to share your song, and we refuse to work with any playlisters who do that.
Has SubmitHub ever had issues with Spotify:
No. We started featuring Spotify playlisters in ~2017, and have had zero issues then because our system is not considered payola.