I'm no expert, but wouldn't it be easier to just use PayPal for commissions? More people have PayPal accounts than Ko-fi accounts, and Ko-fi goes through PayPal anyway, right?
Also:
Another way to do this is to draw the commission, then send the client a very low-resolution and/or watermarked version of the result before they pay. Once they have paid, send the full version. That way they can see what they're getting before they pay you, but there is still no risk that they'll be able to just take the artwork and run - there's verification on both ends.