It seems 入藏邀请函 is just a joke. When I was there a couple of years ago, they wanted to charge foreign passport holders 800 yuan for that, if you get it directly from the Tibet Office in Chengdu. But the price dropped if you buy the airticket (to Lhasa) to a variable amount from 100-500 yuan, which is just an extra charge on top of the regular airfare - there was really nothing called 入藏邀请函 issued. And of course, no one bothered to check my 入藏邀请函 throughout the entire journey inside Tibet. I assume you really do not need that if taking the buses from Germu, which lasts for only 20 hours (to Lhasa), and costs far less than the flight from Chengdu. BTW I was on my own and visted a few other places than Lhasa, excluding border towns, which was said to require the 边境证.