I'm still not sure how that would not meet the definition of an advertisement. An advertisement does not have to be in "mass" to qualify as an advertisement. From 226.2
(2) Advertisement means a commercial message in any medium that promotes, directly or indirectly, a credit transaction.
and from the Commentary;
H. Letters sent to customers as part of an organized solicitation of business.
It appears to me your letter is an organized solicitation offering your customer a special refinancing offering to retain their business.
We don't do a lot of advertising but what we do is primarily generic where we don't use triggering terms, so I am far from an expert on advertising but I don't see how this solicitation would not be an advertisement.
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The opinions expressed are mine and they are not to be taken as legal advice.