OP. you say you have a 7-speed SunRace. That sounds to me like a freewheel, not a cassette. (A freewheel is a self contained unit that houses the freewheel system in a steel cage that screws onto threads on the hub shell. A special tool fits onto the outside face of the freewheel (FW). A large crescent wrench is used on the tool to unscrew the FW. (You pedal it tight. It takes real torque to remove.) The cogs slide onto or screw onto the FW body. Last one or two always screw on and lock the rest in place.
A cassette on the other hand is just a carrier that accommodates cogs that slide on. They are locked on with a cap that screws on with a splined took that fits onto the end. (Same place a FW remover would fit a FW.) Since the cap does little more than keep the cogs secure, loosening it is not hard. The freewheeling mechanism of a cassette hub is on the hub itself, inside the cassette carrier over it. The shop should have gotten you cassette right if that is what you have.
How to tell what you have: almost all FWs have black or grey or dark gold steel where the tool fits. Almost all cassettes have chromed or silver locking caps. Look at your FW or cassette and see what you have. (Report back if you cannot tell and quote me so I see the E-mail that you have a question. Tell us what tool appears t be needed to remove the FW or cassette. Splined or two or three rectangular holes are the most common.)
If you have a FW, what the pawls and springs need is oil. Light, clean quality oil. Doesn't have to be fancy. 3-in-1 works as well as any. Get a good sized bottle. Lay the wheel on the ground, FW up. Pour oil into the groove between the stationary inner of the FW and the rotating outer body. Spin the body and watch for the oil draining into the body. Keep adding oil until you can see oil coming out the back, ie between the FW and the hub flange. Pull a rag between the flange and FW to clean things up as best you can (it hardly matters unless it bothers you to see it) and go ride. My money's on it working perfectly. Repeat as needed, every six months if not. Using this much oil both guarantees everything is well lubed and it flushes any dirt that might have gotten in.
You will find this in any repair manual dated 40 years or more ago.
I went back and reread your post. You are quite clear you have a cassette. You can skip this, other than the advice of good clean light oil on the pawls and springs still applies. I am not a cassette guru so I will let others with more knowledge than I help you. I'll leave this for anyone stumbling on to the thread with a freewheel.
Ben