A billow cape looks like it just allows small creatures to make the following acrobatics check any time they fall.
Falling: When you deliberately fall any distance, even as a result of a missed jump, a DC 15 Acrobatics skill check allows you to ignore the first 10 feet fallen, although you still end up prone if you take damage from a fall
Screw that. Hoping it did more. Thank you. Got a Goblin build riding a giant bat. Decided to be goofy and go the whole bat theme. A Billow cape at first read like a cape used to prevent you from going splat from falling. Go bat cape go bat bomb go bat etc etc.
well, a billow cap, the feat cat's fall and the trait bouncy would let you fall up to 40 feet with relative safety. But yeah if you don't plan on falling very much and you just want something to protect you should you get knocked off your mount.
You'll probably want to either get a Sable Company Elite Saddle, a Harborwing Cloak or even just a few snapleafs
depending on how much you want to invest in it.
Why non-magical?
Snap leafs work great and don't require the user to have any magic, it's just a one time use magical item.
Keep a stash on you and you're good to go.
You're not going to find something that gives you complete immunity to fall damage that's non-magical.
Another great magical solution is boots of the cat. You don't take no damage, but you do take minimal. Maximum falling damage is 20d6, you take 20 (and always land on your feet).