The one Jeff linked doesn't have a removable shoulder strap, but Brenthaven makes Surface-specific products including this:
Collins Vertical Messenger - Indigo Chambray | Brenthaven.com
Edit to add: I can see how vertical would be weird on a motorcycle, but why not a backpack?
Backpacks are pretty much out because I typically go straight from the motorcycle to a meeting at a client's office. While backpacks are very functional, they don't often find themselves in "professional" environments, for some reason. I actually have a wonderful backpack that I picked up in Italy, it's very fashionable and only
slightly too large, but I can't use/wear it where I work, sadly. Unfortunately, I
do work in an environment where appearances count (ties, jackets, Rolexes, etc).
That Collins is actually quite nice, too. That one is definitely in the running, thanks!