What many consumers do not understand is that online agencies do not offer significant discounts from regular travel agents. Most cruise lines have level playing field pricing policies these days. Agents, whether they are from a small office or a large online agency, cannot sell a cruise for less than the cruise line nor can they rebate more than a specific percentage of the cruise fare back to the customer in discounts and onboard credit.
While the above is true of some of the cruise lines (Royal Caribbean and Celebrity), since the OP was referring to Princess it doesn't really hold true. While travel agents can't advertise prices lower than the "standard" cruise line pricing, they can privately discount them as much as they like.
That's not to say any independant agents can't discount an equal amount. If they choose to, they can. But, though it's a bit more time and effort than plugging info onto a TA's website, one can still save $$$ by shopping for cruise prices.