At Admin Panel -> Miscellaneous -> Advanced Options, you can select to make the sponsorship system paid (normal default) or unpaid. Please note that unpaid sponsorship is only available for listing type sponsorship, not usergroup sponsorship.

Unpaid sponsorship means that the user can simply click to promote their listing to the sponsored type at will. Some people want to use this as a way to encourage people to frequent their website. The sponsorship might be set to last for a day, for example, so that the listing owner would have to login every day to keep the listing sponsored. The link to renew their sponsorships will be on the member home main page (the page they reach by clicking "Options"), with a link next to each listing shown in the table.

After changing the advanced option, go to Admin Panel -> Settings -> Sponsorship to set up the sponsorship levels.

Of course, the vast majority of people will only be interested in paid sponsorship.