No idea who Bundle of holding are, but they're doing a deal of a lot of the old MegaTraveller pdfs at the moment.
It's one of those things where you sort-of pay what you want, and a percentage goes to charity. There's a threshold above which you get more stuff, but the threshold keeps going up. I'm sure that makes it crystal clear
Anyways, the deal is
here if you're interested. The player guides start at $5.95 for the bundle, but approx $20 (at the time of typing) gets you the referee's bundle, which has lots of extra goodies.
I appreciate MegaTraveller wasn't the most popular incarnation, but there's plenty of background material for other things in the deal.