There are several ways folks get shipboard credit on Radisson. In no particular order:
1. Your TA may be running a special where they offer a $100 or $200 pp shipboard credit (I've even seen as low as $50pp). My perception is that this is NOT from Radisson but simply the TA offering an incentive to book with them.
2. Any reservation made with an AMEX Platinum Card (the one that costs $350 or so per year) gets a $150pp shipboard credit for up to three cabins so booked. If you happen to have one of these or anticipate travelling on Radisson a couple of times a year this is a REALLY good deal.
Similar discounts are available to return cruisers (5%), book the next cruise on board (5%) (this is a good way to go if you like cruising withRSSC), and through the Gold Points program (basically you earn points for $$ spent on RSSC that are redeemable for a variety of things including $100 off your next RSSC cruise). Your TA should be able totell you about all of these.
PS We've been on Mariner and I truly hope you love it as much as we did.