I've used neither service and only briefly skimmed their websites in the past.......
It has been my understanding from many posts on the US boards that these companies will insure ALL forms of mail as well of course as other type of shiping.
It's possible that there are restrictions on mail which originates from outside the USA but I just had a quick look at DSI and I don't see any. The only mention of Expedited/Xpresspost is this one:
DSI policy holders do not have restrictions based on whether their chosen class of shipping service is traceable or not. However, when ordering coverage for a one time shipment online, you must ship your parcel by one of the carriers and classes listed here.
This means that the restriction does not apply to their "active shipper program" which is what a regular seller would be using as it is much cheaper than the single shipment program.
For specific clarification go to the source.
From outside observation UPIC seem to be the most popular in the USA but possibly DSI provides a better service to non-American sellers.
Ben

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915

"What else could I do? I had no trade so I became a peddler" - Lazarus Greenberg 1915
- answering Trolls is voluntary, my policy is not to participate.