I was having the same issue that many people here have reported. That either immediately or after 4 minutes that I would get the message "Invalid Key or Package name Error". I changed the name on my EasyAR account (was initally incorrect) and the package name to match. (Just to be clear, the instructions talk about a Bundle ID but that is incorrect - it is the package name under Build Setting -> Package Settings -> Other Settings -> Package Name and is in the format com.YourCompany,YourApp and your also have to put it in the EasyAR account under the PackageName ID exactly in the same format)
However this initially did not work and cause me a lot of problems. Eventually I thought that maybe changing the name has something to do with it, so I deleted the SDK license key and then recreate it and then copied the new key to Unity and then rebuilt.
Now everything seems to be working fine. My guess is that the SDK License uses the name of the project to generate the key but then does not update if you change the name and therefore causes an error. Anyway, it worked for me and I thought I would share to help out anyone that was as frustrated as I was.