Hello Nilma! Petsmart and similar store salesmen are there to make a sale, it's their job! Also, stores train employees in order to make sales, period. Many times their amphibian care procedures are incorrect (wrong substrate, etc.). Here is a good article on FBT's care: Frog Forum - Fire-Bellied Toad Care and Breeding - Bombina orientalis and relatives .
A heating light bulb or ceramic emitter of the appropriate wattage will dry the air a bit; but that is the reason we use hygrometers and maintain a proper humidity level for our frogs with manual and automatic spraying or the use of humidifiers. Frogs will not dry up unless you put a high wattage lamp exceeding the enclosure requirements and do nothing to control humidity. By the way, my domes or light fixtures are right on top of screen cover with no issues and have never used a light stand.
If you got the heat mat recently, do not like it, and have the sales ticket, take it back. Heat mats can't be used under frog tanks, are extremely inefficient when attached to an enclosure side, and there is the possibility the glass will shatter if water splashes on a hot glass pane. Good luck!