Also, I think if you were to separate the heat pack from the bag by a layer or two of paper or towel, that might decrease the risk of overheating.
What I might do is this: Come up with a plan, then pack up the box and fish like you planned, but don't send it. Then, try to reproduce the temperatures it might take on its journey- maybe leave it outside from early morning until noon, then check the temp. If it's good then, you could put it back outside and keep checking it throughout the afternoon and evening to see if it ever gets way too cold or hot (a digital thermometer with a remote probe might be useful, you can get them cheap for reptile tanks). This should give you at least an idea whether your packaging is well insulated enough or if your heat pack is too big, etc., and you could adjust before you send it off.
I would probably do what you said and forget about it, especially if it's a $50 pleco