I don't ship. However, when getting my tort and other reptiles shipped too me, I took into account the temps on both ends and if there were any bad weather inbetween.
If its going to be much below 40, don't ship. DO NOT use two heat packs! I made that mistake last year. It was a well insulated box, about 45 out, one four inch hermann. I used two heat packs and the tort arrived well cooked. What a horrible end I put that tort thru.
For a box 15x11x7 and under, you should only ever use one heat pack. Additional heat packs in a small or medium box are more likely to overheat your box, as has been mentioned. Too much heat is more dangerous than too much cold.
A better winter shipping approach is to increase the insulation value of your box. We would typically double up insulation panels for cold winter shipping. Another option is to use the thicker insulated "ice chest" style box, where the insulation is one piece, plus a lid.
I have seen folks line the standard foam insulation with another layer of flexible fiberglass or foil insulation. I can't speak to the effectiveness of that first hand, but it also seems viable.
Heat pack wise, consider using a larger/longer heat pack, like the 72 hour heat pack. It doesn't get any hotter, just offers longer duration, which is a good thing in winter shipping.
You don't want to make your box hot, no more than you want to make it cold during the summer. You want it to be temperate. Feel free to contact us with any specific questions, we are always happy to help!