OK, just a thought...The TA must sell the tariff price....The ticket will reflect the tariff price. What keeps the TA from cutting you a check for say 8% of the value of the cruise?
The is no paper trail for a cruise line to follow.
Or the TA can purchase an onboard credit. Or the TA can pay for a hotel night pre or post cruise. Or the TA can use somem of the commission to discount the air fare.
Lots of options for the creative TA.