+1

If you ever blow a zipper out on a backpack you can punch a small hole on either side and lash it closed.

I've mentioned this before, but if you're going to carry zip ties, go with Thomas & Betts whenever possible, as they feature a stainless steel tang for improved holding strength, and are manufactured to meet or exceed most automotive/aviation specs. You'll want to get the black ones as they are UV resistant, whereas the white ones are not. T&B also offers a variety with an integral mounting head which can be very handy at times (i.e. running paracord through it while lashing a shelter).

Jim