EX Portable 20" Aluminum Guitar Effects Pedalboard with Waterproof Soft Bag, Power Supply, and Daisy Chain Cables - Lightweight and Durable

Q: Does the 20.8” come with Velcro?

Yes, it comes with a velvro and a tigh wire

Q: Does it come with bag?

This one doesn’t come with bag. We have a with-bag version here: https://www.amazon.com/dp/B073QNJG5K

Q: Does anyone have a picture of pedal secured with tie strap?

I use Velcro, available from Amazon

Q: What is the exact clearance underneath from the flat underside to the edge? trying to fit a ernie ball volt (3.5”x2.3”x0.93”)

If your plan is the the volt under the pedal board, at best it's going to extremely close and I don't think it will fit well without some modification. You can probably fit the unit in there, but you probably won't have enough space to plug anything into it. I think you could pull it off if you used different feet than the ones that come with it and raise is up a half-inch or so.