I am with you, bought it 9 hours ago.
I just checked my invoice now and the content indeed modified from "startup plan" into "basic plan".
for the record, when I buy it, it was still "startup plan" and it still showed me that i bought 500k points in startup plan.
I hope someone from mixpanel can answer this, i think it is very wrong to change invoice content like this.
my email: admin1 at poshtech com