I don't have enough experience with EX or XP to the USA to comment on delivery times. I can say that I have had very few problems with Air Small Packets, NEVER having lost one out of at least 2500 sent.
Looking at your current listings, all would be covered by Small Packet insurance, I've only filed one claim (sent to Germany) but it was very easy and fast (but you have to wait 60? days from mailing to file).
Personally the $19.99 would scare me away, I think if you offer a premium shipping service you should also offer a lower price option (but probably not the other way). For the under $80 items at least why don't you offer a "basic" service (small packet) and a "premium service" with tracking (either EX or XP). In general I would expect that SP-Air is faster than EX depending on the destination.
Other things about your "new TOS"...
Canada - Do you really insist on a signature on delivery? That's a pain, my postman would drop off a package with a jersey but I'd have to go to a postal outlet to pick it up if a signature was required.
International - Is that small packet? if yes then US$80+ is insured
USA - Like I said above, 20 bucks is pretty steep.
And trash the "Important Update" it is somewhat negative and sounds like you are trying to justify excessive shipping & handling (which you are not...but going in that direction).
Ben

"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.