The framing industry has a structural incentive to complicate the process of protecting flat paper. Consultations, custom cuts, specialty glass, archival mat boards with competing certifications — by the time you have finished discussing options with a competent framer, you can easily spend three times the cost of the poster on the container. Some of that expenditure is justified. Most of it is not. Understanding which decisions actually matter, and which are elaborations on simpler principles, will save money and produce better results.