In the Antiques RoadShow vein, "Everything is condition, condition, condition". The cover here has significant wrinkling, and being a mass-produced philatelic item (rather than a unique piece of regular mail), its financial value is rather modest. It is not in "exhibitable" condition.
Building on blcjr's post, using
ebay's auction format (rather than the buy-it-now format), one does not have to know a retail value for anything, but instead rely on decent decriptions and quality images. The marketplace will do the rest. Yes, some things will bring less than what you think, while others will astound you as every viewer/collector brings a differet set of knowledge and demand to any given lot.
As for what to keep vs sell, that it up to you and the family, but a large collection amassed by one person should not have to become a burden or obligation for the rest of the family if interest is minimal. I have found that a modest few albums or binders.with a representatve sampling of the collector's main interest is often enough of a reminder yet not an overwhelming burden. And secondly, to get help from a knowledgeable friend/collector to help sort through the collection and direct various groupings to other collectors/dealers. Otherwise it can quickly become a lengthy and time consuming task to come up to speed on someone else's lifetime of work. In other words, for most collections, 95% of the value is in 5% of the bulk. Having some help separate the wheat from the chaff often makes the task a lot simpler.