I had a charge from SMPLYSNUP as well, it was from a local music & art place in NJ where we held a birthday party. They had told me the payment would be processed by SimplySignUp, LLC so it was legit for me.
I was trying to figure out what this charge was for, on my credit card statement, too. Now that I see the mention of Kindermusik, I remembered that we had signed up for an upcoming session. Kindermusik must use SimplySignUp (thanks, Jeff) for its payment processing service.