To start off with, a little context: this tidbit was included by Christie in a list of some "very curious items" (i.e. odd stuff), alongside "Always leave something on your plate for Lady Manners" ("Miss Manners" was the pen name for etiquette writer Judith Martin, popular in the 1970s; here Christie is referring to older and more provincial uses of an eponymous figure who is an archetype of good etiquette) and "Put on clean underclothes when you are going on a railway journey in case there should be an accident." - a piece of advice so hackneyed that comedians in the 80s were still making fun of it.

The matronly nature of this advice is particular: books on etiquette and child-raising really exploded during the 18th century, and traditional child-rearing practices began to be informed more heavily by books of instruction and aphorisms, with more focus on the minor details of etiquette in society...and that's part of what makes this unsourceable; little etiquette tidbits like this tend to be either timeless (clean underwear) or bizarrely idiosyncratic (2 halfpenny stamps)...but I can talk about the thought process that goes into the latter.

Britain during the late 19th/early 20th century was highly class conscious, to the degree that even small gestures were cues to social status - while it was unseemly to flaunt wealth, it could also be unseemly to flaunt economy measures (using two halfpenny stamps instead of a penny stamp implies you can't afford a penny stamp, but are using up your halfpenny stamps) when addressing friends and acquaintances (tradesmen would, implicitly, not be in the same social circle and so you wouldn't care what they think).

Little gestures like that are often invisible in the wider social context unless, like in this instance, someone like Christie deliberately calls them out, but it's all about the grand tradition of keeping up appearances. I can cite one example of "postal etiquette" in this general tradition:

It must also be borne in mind that any suggestion for postage-stamps in 1834 would have been useless, for at that time letters were habitually sent unpaid, it being them considered a breach of etiquette to prepay a letter, just as it would not be quite contrary to usage to enclose (unless to a person of inferior rank) a postage-stamp to prepay a reply to an ordinary letter.

• The Penny postage jubilee and philatelic history (1891)

Which showcases how ideas of rank (socio-economic status) informed such relations as sending a letter; prepaying for a letter at that early date implied that the person receiving it couldn't pay "postage due."