For announcements/advertisements on pieces of paper, I have seen it referred to as "flyer" and "flier". Are half of the people using the incorrect spelling or are these two words interchangeable like "labor"/"labour"?
As indicated by other comments and answers, flyer/flier isn't a US/UK difference like color/colour.
Both spellings have been around for centuries, and for most people, they mean the same. The usage charts for frequent flyer/flier and printed flyer/flier show no significant difference US/UK difference (flyer is 2-3 times more popular on both sides of the Atlantic, for both meanings).
Some authorities claim there is (or should be) a distinction. Note this from dailywritingtips.com...
Interestingly, two American authorities, Bryan A. Garner, author of Garner’s Modern American Usage, and the Associated Press Stylebook, recommend flier for all senses; however, Merriam-Webster’s Collegiate Dictionary allows that flyer is more common when referring to a leaflet, and popular usage bears this out.
Personally, I think that perspective is simply mistaken, as I hope my own usage charts show. What I do find interesting is that among similar forms, tryer/trier and cryer/crier are atypical, in that they've decisively settled on (or retained) the -ier version. Most others, such as fryer/frier and dryer/drier have moved (or are moving) more or less decisively towards -yer.
Thus the preferred orthography is currently unsettled, but flyer seems to be following the pattern whereby noun senses (typified by the plural forms dryers/driers) favour -yer, and comparative usages (such as is dryer/drier) favour -ier. It's enough for most writers to recognise and respect that emerging consensus - two different noun senses is just too fine a distinction to survive.
Flier e is a variant of flyer and they have been both used for long:
Flyer also flier, mid-15c., "something that flies," agent noun of fly (v.1). Meaning "something that goes fast" is from 1795; that of "aviator" is from 1934. Meaning "speculative investment" is from 1846 (on the notion of a "flying leap"). Meaning "small handbill or fly-sheet" is from 1889, U.S. slang (originally especially of police bulletins), on notion of "made to be scattered broadcast." Meaning "aviator" is from World War I. Related: Fliers; flyers.
While labour vs labor refer to different spelling ( British vs American)