A mezuzah is really two objects treated as one, a small handwritten parchment scroll and the case that holds it, and Jewish law cares a great deal about the first and almost nothing about the second.
What is sacred, and what is decoration
The parchment, klaf, is written by a trained sofer using the same exacting standards applied to a Torah scroll, containing the Shema and a following passage, and it is this scroll alone that carries sanctity. The case can be wood, metal, ceramic, glass or plastic, plain or elaborately decorated, bought at a gift shop rather than commissioned from a scribe, because the case is a container rather than a ritual text.
Who affixes it, checking, and what happens when it fails
Any observant Jew may affix a mezuzah to a doorpost, reciting a blessing as it is fixed at an angle near the top third of the frame, and custom calls for checking the parchment periodically, commonly twice every seven years, since damp, insects or fading ink can render it unfit without the case showing any sign of it. A parchment found posul is not thrown out with the case, it is treated like any worn Torah text, stored and eventually buried, while the case itself, carrying no sanctity of its own, can simply be reused with a fresh scroll inside it.