I'm already registered through the Streamlined Sales Tax Registration system (SSTRS), can I start usinga Certified Service Provider (CSP)?
Answer:   You may contract with a CSP after you are registered.  Contact the CSP you wish to use.

If you registered through the Streamlined Sales Tax Registration System (SSTRS) prior to having a contract with a CSP, you are responsible for filing the returns and paying the tax directly to each state, until the contract with the CSP begins.  The CSP will file the returns and remit the tax for you once the contract has begun.