how can I insure a shipment for more than $10,000

If you have your policies with high net worth type insurers, e.g. Chubb, Pure, they can write riders that will cover your while gear is in transit. I did this when I moved from NY to Texas because the mover’s insurance is based on gross weight of load and is largely worthless. I wound up keeping most of the riders in place, but there is no reason I can think of why you can’t discontinue the coverage. Usually, such companies want all your business- home, autos, valuables + umbrella coverage. I found this years ago when I bought my first exotic car- the normal insurance companies wanted a fortune to cover the vehicle. I got to Chubb and it was a fraction of that and I shifted all my coverage to them. Unfortunately, Chubb’s rates are quite high, I think in part due to the wildfires in California. We now use Pure, which is a similar type of insurance. But this means changing insurers. I doubt they will write a policy as a one-off if you aren’t a client. 

PS: I'm not in the insurance business and somebody who is can probably answer your question more fully including whether any companies will write limited term insurance without being a regular client. 

Good luck, 

looks like DHL and FedEX will go higher if you book direct rather than through a 3rd party like PayPal ship or Pirate Ship and others.

I think, USPS Registered Mail could be insured for up to $25k, unless they changed it. But I have no idea about either the cost or weight limits or etc. Registered Mail is also very slow, but very reliable.

They mail diamonds all the time at the USPS. 

Guess I'd be focused on finding which either insurer or shipper actually stands behind the insurance if a claim needs to be filed; how the items needs to be packed and by whom with proof of such; what needs to be in place as proof of item and packaging condition prior to shipping, and what's required afterwards. Nothing is ever straightforward in these matters