Once a bounty is accepted by a dev, the initiator of the bounty opens a private lightning channel to the dev with the bounty amount. (This shows that the initiator has the funds and is committed)

Open source the code from the beginning/ or have milestones. (This to show proof of progress)

After each milestone, a certain amount is pushed through the channel. (This helps the dev manage the risk, if he doesn’t receive partial payment they don’t continue). 

Once the amount is moved, channel is closed. 

The fees are security/risk mitigation costs.