There is no such thing as a BIFL waterproof backpack.
Buy a BIFL pack with a good aluminum internal frame, and then also add to your cart:
- A waterproof cover for your pack
- A pack of very thick (construction-grade) garbage bags
The garbage bag is your pack liner. Leave it inside the pack at all times. It keeps everything inside your bag dry.
To keep the pack itself fairly dry in light rain, you can put the waterproof pack cover on the outside. This will just make your pack quicker to dry when you get to your destination.



I recommend buying a brompton bicycle.
The rack being super low means the distance between the rear rack and the saddle is almost the exact height of a good backpack. IMHO, it’s the best way to carry a backpack on a bicycle
Edit: Brompton’s small front O-bag is actually made by Ortlieb. I wouldn’t recommend their bags as backpack replacements. They work great on bicycles, but not so much on your back (where the weight should actually be on your hips)