Hey Greg!
Use Bonus Bet(SNR) mode for those. For risk-free offers we use the SNR mode even for our first bet to ensure we make money regardless as it is risk-free. So use SNR mode for your bonus bets at higher odds.
If you ever confused with whether a bonus bet is SNR or not, check the terms of the offer to see if it states that your winnings from bonus bets are stake returned or not. They usually are not, hence SNR(Stake Not Returned).
Hope that makes sense!