I mentioned the Celebrity single penalty. Here is one that
Travelocity and Princess have collaborated on.

They are offering a $200 shipboard credit on select sailings.
Here are some details.

"...Single occupancy staterooms will only receive half of the
applicable Onboard Credit amount and only one per stateroom may be
applied..."

"...Based on double occupancy or better. Single guests must pay 200%
of the cruise fare to be eligible for this Offer..."


I have to pay twice the fare but get only half the shipboard credit.
It appears to me that the cruise lines really don't want singles on
their ships.