Sadly, No alternative for CoinPayments that I'm aware of so you will most likely need to build your own API and run full nodes of these coins which could cost you much money. Rocketr and selly are just sites to sell your digital goods for bitcoin.
I understand, I don't think they use Coinpayments though and I doubt both have created their own API for them both.
What is the issue with coinpayments?
You could use the WAVES platform to accept all of those (except from dash)
What's the waves platform? And no issue, just would like to use something else.
Edit: Looked it up, looks interesting, I will look into it more... thank you for the suggestion. If I'm to use them, any alternative for DASH?