When I am considering shipping temperatures, I am looking at the daytime highs. Unless there is a cold snap coming, some crazy unusual weather, night temps will fall in line, generally about 20 degrees lower than the daytime high.
A well insulated box protects you from temperature swings, just as much as it contains the heat of a heat pack.
Daytime highs of 80's in one spot, and 70's in the other? Night time lows of 50s? Ship in the insulated box with no heat pack, every time.
If you want to check the temp variance yourself, include a Min/Max digital thermo in the shipment, and have the recipient check it upon opening the box. We recently did a new temp test with a box shipped during overnight lows of 30's, and with the heat pack, it never got below 69F.
Even without a heat pack, being exposed to outside temps in the 50's, I bet the interior temp never gets below 65 or so.
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