how burial containers and other funeral goods are priced, sold and regulated when a family is arranging a burial or cremation
The grave liner requirement comes from the cemetery, not the funeral home, and the same grave can be quoted three different ways depending on who sells you the box.

The outer burial container is the one item in a funeral bill where the requirement and the sale come from two different places. The cemetery decides whether a grave gets a liner, a vault, or nothing at all, and it writes that decision into its rules and regulations long before you walk in. The funeral home sells you the box, or a third party does, or the cemetery does. Three sellers, one requirement, and prices that can differ by more than the casket did. Here is what a careful reader checks before signing.
Call the cemetery office directly, not the funeral director, and ask for the section of the rules and regulations that covers outer burial containers. Most properties have this as a printed handout or a page on a website, and the ones that don't will read it to you over the phone if you ask them to put it in an email afterward. What you want is the exact language: whether a container is required, whether it must be concrete, whether a sectional liner qualifies, and whether the cemetery charges an installation or setting fee separate from the container itself. Get the person's name.
A grave liner is a plain concrete box, usually with a flat lid that rests on top, sometimes made in sections, and its whole job is to keep the ground from sinking as the earth settles. It is not sealed and does not claim to be. A burial vault is a heavier container with a joint designed to resist water and soil intrusion, often lined with plastic or metal, and sold with a warranty that describes the seal in careful terms. Both satisfy a cemetery rule that simply says an outer container is required. Only one of them costs what a small car costs.
A funeral home shows you a vault selection room with four or five models arranged from plain to elaborate, and the plain one is often not on display. The cemetery, if it sells containers, quotes you a liner as part of an interment package, sometimes bundled with the opening and closing fee so the individual price is hard to isolate. A third-party dealer sells the same manufacturer's product delivered to the cemetery for a delivery fee. Same grave, same requirement, three prices, and the gap between the lowest and the highest is frequently larger than most families expect.
The Federal Trade Commission is responsible for the rule that governs how funeral providers disclose their prices, and it requires an itemized general price list along with a separate list for outer burial containers. Ask for that container list on paper and take it with you. Check whether the price includes delivery to the cemetery and setting at the grave, because some homes quote the container alone and let the cemetery bill the setting separately. Check whether the cheapest item on the list matches anything actually displayed in the room. Ask what the unlisted models cost.
Sealed vaults come with written warranties, and those documents are worth the four minutes it takes to read one at the funeral home rather than at home afterward. They typically promise the container will resist the entry of outside elements for a stated number of years, and they typically say nothing at all about the condition of the remains inside. That distinction is the entire difference between what a warranty covers and what a grieving family hears. If the seal matters to you for reasons of your own, buy it knowingly. If it doesn't, a liner meets the same rule.
Before you commit, write out the container price, the delivery charge, the setting fee, and the cemetery's opening and closing charge as four separate figures, and have someone at each business confirm their own line. Families are often surprised to learn the setting fee belongs to the cemetery and appears on a different invoice entirely. Once you have the four numbers, call the other two sellers and price the same combination. The requirement will not change, because it was never the funeral home's requirement to begin with, and that stability is exactly what makes the comparison honest.
Cemetery rules vary property to property, and a rule that seems arbitrary usually traces back to how that specific ground settles under maintenance equipment. Ask, read, and keep the paper.