Your understanding of the promo is wrong. They will give you double winnings on your first $30, not upto $30. So Richmond odds were $1.25, then max bonus is $30 × $1.25 = 37.5. But its just double winnings, so $37.5 – $30= $7.50.
You should use this promo on the underdog as the winnings for the first $30 staked will be more than on the fave.