I don't think the USPO has special rules for numismatics. I think they insure them like everything else.
Until a year or two ago I don't think you could get REGULAR POSTAL INSURANCE for over $600. You had to use registered mail.

Here is the main reason for my post here: It is cheaper to use registered mail insurance for high priced items. I use registered mail for items over $600, I believe. The greater the value, the greater the advantage in using registered mail insurance. (Note, that one slight hassle is that the registered mail package or letter can not be sealed with plastic tape; you must use paper tape or glued paper to enclosed the parcel.) For example, the cost of $1000 insurance with registered mail is $7.30; it is $10.35 via regular postal insurance. The disparity is greater with higher insurance.

Robert