It's not uncommon for civic-minded businesses like your bank to publish congratulatory letters in local newspapers for school groups and others who don't often get the recognition they deserve in the press. Sure, it gets the business's name in front of a lot of eyes, often at a lower rate than the typical ad occupying the same number of column inches, but it also echoes the support that others are offering for whatever achievement is being celebrated.
Would I categorize such efforts as an ad? Not if it doesn't "pitch" the bank or its services. As you have described this gesture by your branch manager, I don't see it as a pitch for business at all. I wouldn't worry about adding "Member FDIC" to the letterhead in this case (it would not be a violation to include it, however).
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John S. Burnett
BankersOnline.com
Fighting for Compliance since 1976
Bankers' Threads User #8