Hmm, okay. There's a bit to break down here. To be frank, the unfortunate reality is that most pet stores don't care about spreading correct information. For many, it's about making a sale (thus with the "corn snake kit"). The enclosure itself is doable, but there are definitely some changes.
1. Your heat mat NEEDS a thermostat if it doesn't. If they didn't tell you, that's absolutely essential. A heat mat not connected to a thermostat can keep getting hotter and hotter. It can potentially burn your snake or, worse case scenario, kill your snake (from long term exposure, short circuiting and causing a fire, etc.).
2. Get rid of the thermometer they gave you and get a digital one when you can (and a digital hygrometer; many have a 2-in-1). Analogs are inaccurate, so it's possible your temperatures and humidity are too hot or too cold. If your temperature is too hot, it could be drying out the enclosure. Simultaneously, your humidity might just be too low.
3. I admit I'm not 100% sure about the substrate. I've heard it's more popular with colubrids (but as I mentioned, I'm more into the ball python community). That being said, I would first try covering the entire top of the enclosure with something like foil tape, a towel, or plexiglass. The slits on the side are enough for ventilation, and covering the top will help stop humidity from escaping.
-fireflii