I appreciate the responses and suggestions.

I agree mailing a heater box is risky. I sent off my original 69 Roadrunner unit to a guy about 10 years ago to be restored. When he received it, he scolded me for not packing it well and said he was shocked in arrived in one piece. When he sent it back, he built a wooden frame that surrounded the box containing my freshly restored heater box. USPS managed to crush the wooden frame and crack the heater box housing and break the resistor plate. Go figure!