Why don't you use your own non-custodial wallet and send coins directly instead of using Binance or other centralized exchanges?
This way you will probably pay less fees and you will have more control over your coins.
I was also thinking that's the best way to pay for the membership from your own wallets but maybe he is more comfortable using the exchange directly to pay for it along with the fees if he use Binance or Kraken.
If you want to save on fees from exchanges than try using FTX, I think they are not charging any fees for withdrawing Bitcoin (on mainnet).
I think only small bitcoin withdrawals of 0.01
BTC are free but you have to pay fees for amount exceeding this withdrawal limit so it's free for paying copper membership of forum.
Small BTC withdrawal fees: BTC withdrawals > 0.01 BTC are free. BTC withdrawals < 0.01 BTC are charged withdrawal fees after your first free one per day.
FTX withdrawal However if you have staked FTT on their platform then you can also get some free fees waiver per day on ERC-20 tokens which you can check on their help page.