How to trigger additional paypal charge with theme custom fields? Pointfinder

Hi,

I am using pointfinder theme for a classified theme. The theme is good. However, I would like to add more more options so that there can be options to charge more. For example, I would like to charge extra amount for the advertisers who want to add their advertisement in front page carousel slider. The theme gives an option to add the ad to carousel upon a custom taxonomy selection. But I am not getting any way to add amount to the paypal once user checks the taxonomy field.

Please see this website and try to post an ad: https://keyspectra.com/ability-classifieds

I hope some hook or anything related could be a solution.