Congratulations on starting up your bike commute! I dislike the weight of a backpack on my shoulders, so my solution has been to use a messenger-style bag with a waist strap. Currently I use a North Face Base Camp, and it's pretty enormous, but the advantage is that I can fit everything in it. The waist strap helps me balance while riding and really keeps the weight almost completely off my shoulders, making heavy loads far more bearable. Good luck!