may be you should name it tokensale, not airdrop. i think airdrop is giving away the token for free.
We are collecting fee only to avoid duplicate/ fake entries, and those anti-spam fees will be used later to distribute coins among all participants!
You sale ICO for 4,000 x 0.002 ETH = 8 ETH, regardless of the amount of ETH you collect. What is a crypto airdrop? A crypto airdrop is when a blockchain project distributes free tokens or coins to the crypto community.