Those products like the commercial reptile heating pads have to include disclaimers for liability purposes, because undoubtedly some fool will figure out a way to burn their house down with them. They should not get hot enough to melt plastic usually. Naturally, don't take a UTH rated for a 200 gallon enclosure and place it under a 18gal plastic tub. That may have bad results.
If in doubt, set it up and monitor it through the day when you are free. Rubbermaids and Sterilites melt around 320-400 degrees Farenheit from the best I have seen. If a reptile heating pad is getting that hot, there are bigger concerns. If you can hold your hand to it for a period of time without being burnt, the plastic should be fine.