misha-
whatever you want to do is obviously your business, but i have to say that i think "refusing a delivery" on a custom bag is pretty bad form. I think you are much better off to either

1. sew a clip on yourself-- it's not that hard. use a key ring if you don't have a D ring. it's not that big of a deal
2. sell your XS on craigslist and buy a small


As for the carabiners, really, you can buy them even in drug stores. the five dollar/seven dollar ones you were looking at from REI's website are going to weigh enough to pull your pants down, if clipped onto belt loops. Maybe not literally, but they are very heavy.

as for tucking the slack, even the XS timbuk2 has cam buckles.

lastly, there's a reason why xs timbuk2 don't have clips for cross straps-- with the little capacity it has, if worn correctly, you really don't need annything to keep it inplace.