There's no such thing as paint sealant. The closest thing is the plastic sheets made by 3M and sold under the name of Body Armor (I know there are other names, but can't remember them). What you may be referring to is the "top coat". Which is basically a thin spray applied at the auto plant over the painted surface that gives it a shine. It hardly protects against anything such as stone chips, scratches, etc.
If you look along the rocker surface (the part of the outer body underneath the side doors), you will notice that the paint is bumpy. There is extra protection sprayed on to prevent stone chips from breaking through to the metal.
The underspray they are trying to sell you will help prevent stones from banging against painted surfaces. Afterall, a second spray doesn't hurt anything. However, depending on where you live, this only lasts 1-5 years and must be applied again. If you don't live in an area that applied salt on the road to melt ice, then I would advise not to get the undercoating.
To my knowledge (having been on many product launches for the Big 3), there is no fabric protection as part of the material used in seats. There are evaluations on the durability of surfaces, but no stain protections.