Actually, the customary reply is "you're welcome", which is short for "you are welcome," which in turn is more or less short for "you are welcome to the service I have just provided."I say "Thank You" and the correct response from the employee might be "your welcome". But NOOOO they say "No Problem" what the heck does that mean?!? I know its no problem, because that is why your employer is paying you , or why I am going to tip you. I am tempted when they say no problem to reply "I think you meant to say "your welcome", but it would probably be lost on them.
I don't usually like to correct spelling, unless there's humor to be had or a point to be made. In this case, there are two points. One, the original meaning of "you're welcome" has been lost, which is one reason people have such trouble with the spelling. Second, language evolves, sometimes in meaningful ways. "No problem" is a perfectly acceptable response to "thank you", with the extended meaning of "It was no problem for me to have provided that service." Another acceptable form of response is "it was nothing" or "'twas nothing", meaning "the service I provided took little effort, making me glad to have provided it." Insisting that your response is the only acceptable response is worse than people correcting spelling in casual discussions, where at least there's often a measure of objective correctness.