This assumes that the price you pay when booking through PTS is no higher than you would pay if you booked elsewhere -and- that you cannot obtain the same amenity booking elsewhere.
As long as you have the Plat AMEX - you can book through any agent (not just PTS) and actually pay with any other AMEX to get the benefits. We got the Plat AMEX pretty much b/c of this since we cruise on Regent once a year - and have done exactly this 5-6x without any problems in getting the $300 Shipboard Credit (additionally our agent usually has more credit along with Regent - it's actually getting ridiculous in trying to spend the credit now that it's all-inclusive including shore excursions). Our agent has a record of our Plat AMEX number but we pay with our DL AMEX.