I’ve semi-monitored it the last several events and it seems to be both of those. It will bump to the $10 pack once you level past a certain step or it will auto-change after the 3 day lime limit.
I realized this when I had an awesome 2 day pack disappear a few events ago that included 10 expansion permits or something insane. It was on my screen with “2 days left” and then it disappeared a minute later…. I think someone confirmed this for me awhile back on the forum but I forgot to ask if anyone knows when it switches.