I've bought a couple of things from them, the latest one being the 70-300 VR lens, which came with the international warranty that you're talking about Soup. Assuming it will be the same, mine was with (
Mack Camera & Video Service | Official Camera / Electronic Extended Warranties in NJ, NY, USA and worldwide) which is reputable I believe. I read the guarantee card but disregarded it, because the company is USA based and, like you, I thought it would be more hassle than anything, so I didn't bother registering. I later learned that I should've, as my lens would have been fully covered locally, i.e. would not have to send the lens to USA in the event of a claim, after all
It was good cover too "7 year lens under $1000" (which this lens was)
Check out Mack for yourself, but I'd say "go for it"
EDIT You could always email Onestop to ask any questions you have, they are usually good at replying promptly