I like space covers that have autographs, especially the ones with the astronauts' autographs. The problem with collecting them, though, is the majority are autopen signatures that look just like real autographs but were made with a signing machine that was programmed to write an autograph with an actual pen and used a template made from an actual signature. Back in the day there were so many requests for autographs that the astronauts were not able or in some cases were not willing to sign them all. There are reference books that show many of these autopen signatures where you can check to see if an autograph on a cover is from one of the several known versions of a particular astronaut's autograph made by one of these machines. The other challenge is many of the astronauts had secretaries and assistants whose job was to hand sign autographs of the astronauts so the astronauts themselves would not have to do so. These tend to be harder to spot unless the secretary or assistant happened to not be trying to make the signature in the style of the astronauts actual signature. The autographs from non-astronauts, such as mission scientists and other non-celebrity personnel are typically authentic as the demand for their autographs was normally fairly modest and tended to be mostly from the die hard space enthusiasts who studied every single aspect of a mission.
Decade of Achievement first day covers tend to sell in the range of $1 to $2 is they are in perfect, unaddressed condition. Wholesale they are a fraction of this. Your cover is addressed, and not very neatly, which makes its commercial value close to zero, but at the same time since it is addressed to your relative who created it back in the day it is priceless to you and your family.
I do not recognize the signature on the enclosure - is it Bill Englert or something like that? I do not know of any astronauts with a name along those lines. As such it would not add any value.