We seem to have terminology issues. What visa have you got (or are getting) ? Visa on arrival is for specific nationalities and is only for 15 days. Visa exempt gives 30. A VOA is not extendable. A visa exemption is for 30 days.
a tourist visa is 60 days and you obtain it before arrival in Thailand. Visa exempt you get for free at the airport on arrival and is good for 30 days. Both can be extended for an additional 30 days (no insurance requirement to extend)
Great to know! I have a 30 day visa exempt starting June 2nd and I thought I would have to buy another month's insurance in order to extend an additional month.