30 day sale vs Until Cancelled

[rmj8781_7]

I'm a new seller with a question - I have placed about 50 items on eBay ranging from $5 to $65. I used the 30 day purchase option with make an offer. I have sold quite a few items. I then noticed there is a "unitl cancelled" option. I checked the fees and they are the same for both the 30 days and until cancelled.

Questions-

I tried to edit the proir entries from 30 days to until cancelled - it won't let me - After the 30 days is up, what happens?  Can I renew the sale, and if so am I charged another incertion fee?

Any advise would be helpful - THANKS