As others have noted, an electric heating pad is not allowed. It is a fire hazard and there may also be issues with electric system compatability [ships have some unique requirements and issues with their electric system; even though it seems like you are plugging into "household power", you are not].
I often use an electric heating pad at home, but I never travel with one.
Instead I travel with ThermaCare Heatwraps in different sizes. I also use them at home and work.
The principle is similar to those little instant hot pads for hands and feet that one might use in the winter. However, these are larger, last much longer, and are designed with adhesive or velcro-like wrap depending on the style so they can be worn while you do things.
https://www.thermacare.com/heat-wraps
Costco and
Amazon and Walmart and probably your local pharmacy sells them, though they may not have all styles.
I suggest trying a size(s) out to make sure it works for you before buying in bulk.
Each heat disc is designed to heat to about 104°F (40°C) for 8 hours. I find they stay usably warm for longer. Obviously at that temp not a fire hazard. Make sure you read the instructions about safely using them !!
FWIW, I can't put the adhesive on my skin as it causes a rash, so I put it on clothing instead. A lighweight bamboo scarf is excellent for holding the neck pain one against one's neck.
SW