It looks like you can type in the amount, but because the box is so narrow you can't actually see what you've typed in.
I'll let the programmers know.
In the meantime, if you type in a number you can use right click to select all, then right click + Copy and then paste it into the comment box to confirm the amount (you can also use ctrl+a and then ctrl+c). Let me know if you have any questions. :)
Try now. Just made it so that the width is hard set (vs just being a % which might be marking it as really small)
Try now - I forgot to push the change live