Red
I'd say that it depends on what you intend to do with the rifle, selling it or keeping it for your own use.  If you intend to sell the rifle, the letter could indeed increase the value of the rifle thereby making the $150 a good investment.  If you plan to keep it, how much is the curiosity you have concerning the rifle's origins worth to you.
Phil

I try to letter every original arm I acquire, as SOP. I've spent some fairly major $ (for me anyway) over the years but have never regretted it and have sold several for noticably higher prices as a result, and in addition enjoy the satisfaction of having the provenance on the ones I keep.

However in this case, if I were in your position, I would try to be happy with simply having the knowledge that the arm is righteous, and instead spend that $150 on My Bride.

Reasons? IMO the knowledge is the most important component of what Dr Labowskie is selling, and he's already given you that for free. BIG DUH on his part! Actually I'm kinda surprised, given my previous acquaintance with the fellow, that he would actually give away anything but a hard time, but apparently you lucked out. My Labowskie letter looks pretty shabby when compared with my two letters from the previous Sharps records owner (Dr Moore) or any of the letters authenticating Colt, Winchester, L.C.Smith, Marlin and Springfield arms. Mighty shabby-looking indeed, and not worth it IMO in this case where you already have the knowledge.
JMOFWIW, good luck, Joe