Grace & Peace!
I’m not sure if this particular practice is Amish, but the sympathy between the Amish and other sects (such as the Shakers or others like the Oneida community, or some more obscure Pennsylvania communities) may partially explain this practice, if indeed it is Amish related.
Certain German theosophical communities (influenced by Jacob Boehme, for instance, the Lutheran mystic who was fond of alchemical and Qabbalistic imagery in his work–which is fascinating, very faithful, and incredibly edifying, by the way) were given to practices which hearken back to European magical practices (one of which, for instance, involved engraving metal plates with names of angels and related prayers in order to protect buildings from fire–apparently to good effect, if the accounts are to be believed). Hoodoo, as it is explicated in Johann Georg Hohman’s 19th century text “Long Lost Friend” (Der Lange Verborgene Freund) is a text which details some of these folk magical practices. While some of these practices are certainly related to the European grimoire tradition of occultism (such as the Solomonic tradition–the Greater Key of Solomon and the Lesser Key or Lemegeton), others are more closely related to Catholic folk surperstition which finds its way (through Lutheranism) into German/European folk custom. Examples of such superstitions would be similar to the use of the Agnus Dei (a wax seal which bears the image of the Lamb of God blessed by the Pope at certain seasons) to ward off lightning strikes when properly affixed to a building.
A more familiar face of this particular tradition are the “Pennsylvania Dutch” (Amish!) Hex Signs–colorful geometrical designs often painted on barns or homes to ward off evil and/or attract blessing. This design is meant to ensure abundance and goodwill:
Under the Mercy,