I have mailed certified and non-certified checks for large amounts when I used to trade physical metals. Many transactions, many checks. Never lost a check or experienced fraud. I know it's possible, but it's not something I'd personally be worried about. If a check gets lost or cashed by a thief, it's not your problem. They will issue you a new check and conduct their own investigation.

Anyone withdrawing cash from a trading account can do a free ACH transfer and have the money in your bank account in 2 or 3 days.

You can also do a wire transfer for a fee that will hit your bank the same day. However, if you have a big ole lump of money in your account, and your positions have significant value, I'm sure they will waive the wire fee.