create the plugin for my website

i have a website made of osclass and i want to add some extra function to it. currently the item post page in osclass does not have premium add selection during the post ad page, and it can be shown in item post page ( by following the steps specified in {[login to view URL]} with payment plugin installed. what i want is you should make these changes as a plugin so i dont want to apply the code every time i update by script and also with some features additional to it, { currently the payment plugin is having a single price pack for all categories but you need to modify that so that i can give price for each category separately with duration for 15 days and 30 days.