Great tips. I do a good bit of that, so seems I'm not doing to badly there. But I will never use a larger box than necessary again. I even cut it down hoping it would not be classified as larger than standard. Obviously didn't make it. If a bass has movement I will place the body in a t-shirt or other soft material to protect the finish. I don't use anything plastic near the finish. Same with the headstock if needed. I always ask about detuning given it seems to be a preference, and tension or not in a HSC shouldn't be much of a factor, I wouldn't think. I personally don't want it done. Necks are designed to hold positive tension which is their normal state for life, and as far as In know, instruments ship tuned from factories. Maybe wrong on that. But I understand some prefer it, thus I always ask and will do what the buyer wants. On a sale, I always state basic shipping, if the buyer wants more than that, they can ask and pay for that. But in your situation I would definately insure the item.
Oddly enough the lady said with the extra size it qualified for $100 of insurance if I wanted it. I politely laughed and said that's fine, but that doesn't even cover the cost of shipping let alone the contents of the box!