My name is Aaron, I'm a guru here at Shopify!
The charge for a paid theme is one time only! Once paid, you own that theme and will not have to make any additional payments. The only exception to this is that the purchase is for one store only - so if you were to open a second store and wanted to use the same theme, you would then need to purchase a second copy.
Hope that's helped! Please feel free to give us a call or start a live chat at any time, we're open 24/7 for your convenience and always happy to assist!
Aaron | Shopify Guru