But if the point had been to sell stamps, wouldn't they have expected to sell MORE stamps if they called attention to the fact that a sheet cost $8.88? But they didn't....

There is some mysterious politics going on here. Like, why did the USPS choose to call it a "Lunar" New Year stamp instead of a Chinese New Year Stamp, although it's clearly a Chinese-themed design? (Jews and Muslims follow a lunar calendar, too, but the design isn't Jewish or Islamic.) You'll see that China isn't mentioned until the 7th paragraph of the news release.