While I do not have much experience shipping from Thailand to USA, I have extensive experience sending both commercial goods and personal shipments from US to Thailand. And the occasional commercial shipment back to the US.

My overall experience: Commercial shipments, via DHL (usually cheapest), FedEx, or UPS are quite easy to do. I did not run any pricing, but $100 USD for 21 kg sounds like a bargain to me. The thing that makes commercial shipments work is that both entities usually have a broker at each end who facilities custom/duty clearing and will intercede with the carrier on issues that arise. Note: There always seem to be issues with DHL - in both directions.

But my experience with personal shipments to Thailand has been an unmitigated disaster. I have tried DHL and US Mail (USPS). None of the shipments ever reached the recipient. The ones I mailed simply disappeared without a trace sine there is no tracking provided once the package leaves the US. DHL, on the other hand, kept sending cryptic messages that there were import documents needed. It appeared that the package was opened for inspection and basically ransacked. Because I was shipping to an individual, who was not represented by a broker, there was no one to force a resolution of the delivery issues in Thailand.

Right now it's hard to imagine a situation that would prompt me to ever try a personal shipment again.

As to your specific query about sending 21 kg. That is a heavy box and even sea cargo is going to be expensive, a black void of no tracking, and completely unpredictable. Do you have any traveling colleagues that might be able to help? If you are in Thailand, you might look for a shipping agency and see what service they could provide. Or talk to a company that routinely ships tourist purchases (I'm thinking antiques, statuary, large artwork) and see what advice they might have.