I have three popped discs on my lower spine. I've been riding with a 10 lb. pack for years
with no problem. Occassionaly, I'll use a rubberized duffle bag with shoulder straps.
I'll load it with 30 to 40 lbs. of gear and peddle 5 or 6 miles. It could be that I'm kinda
muscular and my back muscles support my spine.
I don't think commuter packs get much ergonomic design for hauling a lot of weight.

A hiking pack depends on the hip belt to support the weight. A sternum strap keeps the
shoulder straps from sliding on your shoulder. A decent shop will size you with a pack
that fits the length of your spine for a more efficient fit.

If the commuter market increases, a better pack may appear.
I sure hope that I don't need it.