Is Romans a "bread and butter letter"?

When introducing the Epistle to the Romans in my class on the Life and Letters of Paul yesterday, I mentioned the following quotation from J. Paul Sampley, one that I once used in an examination question:

It is an apostolic response to ethnic problems in those churches, and it is a “bread-and-butter” letter written in advance of his arrival, seeking support for his mission to Spain.

I asked the class if they knew the expression "bread and butter letter" but none of them did. As I tried to explain it, I realized that I had always heard the expression used after a visit rather than ahead of one. I think of it as a thank you letter, written to your host, traditionally, the woman of the house. I looked up the expression in a variety of places, through the ease of Google, and it seems that the usage I am familiar with is indeed the standard. An article on The art of the thank-you note, for example, has the following:

A further subset of the thank-you note is the bread-and-butter note, a letter written after a stay at someone's house. While the specific origins of this expression are obscure — members of the Writing Center at Princeton University were at a loss — "The Dictionary of Slang and Unconventional English" dates the term's appearance in American vocabulary to the turn of the century.

As might be expected, Ms. Post believes the bread-and-butter note is a must, unless the host or hostess is a family member or close friend with whom the guest stays frequently. And her final word on the matter is: "Never think, because you cannot write a letter easily, that it is better not to write at all. The most awkward note imaginable is better than none."

The "Ms. Post" here mentioned is the author of Emily Post's Etiquette: A Guide to Modern Manners. Her book, written in 1922, is all available online and is very enjoyable reading, the relevant section for this topic being Notes and Shorter Letters.

As far as I can tell, therefore, the twentieth century usage of "bread and butter letter" is not used to describe a letter of introduction like the Epistle to the Romans. But did the ancients write bread and butter letters? Mary Johnston says not:

The bread-and-butter letter, as we call it now, does not seem to have been required from appreciative guests after visits. Horace addressed an Epode (III) in complaint to Maecenas after the garlic at dinner had disagreed with him, and Catullus wrote to Licinius that he could not sleep after their poetic contest over the wine. Pliny's letter to his mother-in-law (I,4) was written after a visit to her villas in her absence. Whether Caesar wrote to thank Cicero for his hospitality at Puteoli I do not know . . . ("Hospites Venturi", The Classical Journal 28/3 (Dec. 1932): 197-206).

In short, it seems that the term "bread-and-butter letter" is not ideal for describing the Epistle to the Romans.

Note: I can't find my reference for the Sampley quotation above. If anyone happens to know its location, I would be very grateful.