Micki, as others have said, each 30-days of service is to be paid in advance and is $15. Unless you have a cell phone or another alternate phone, you'd want to keep this service uninterrupted for emergencies, etc. as this is intended to be you main home phone service. And yes, this is the least expensive Straight Talk plan (either mobile or home phone).
AND, as mentioned, you will lose your phone number if 30-days passes after your Service End Date and you have not added (renewed) service as per the T&C of Service. Therefore, although there are no termination or re-activation fees, it is impractical to not to renew service if you want to keep the same phone number.
There are only two plans; one for domestics calls and the other is for international call. Both plans are good fot 30 days.
You can use the ST Home Phone for as many days of the 30-day service card as you want, then not use the phone for as many days as you want and just buy another service card.
Only thing to worry about is losing your prorted tel. number if no service for a long time... don't know how long that is, so call them to check.