"flyer" vs "flier"... interchangable or totally different meanings/usage?

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.