PHP custom coding required for joomla component

Hi I'm using docman seller plugin that allows me to sell documents in the docman component for joomla. At the moment the docman seller plugin allows me to price and sell individual documents in the document management component. I need to add an extra feature that will allow users to buy an annual subscription that allows them to download as many documents as they want from the docman component. For example at the moment a user will navigate to the document he/she wants and will see a buy button at the bottom of the description with the price for the document. What i want is to allow users to buy an annual subscription say for $20 and then their permission level will be changed and the buy button should disappear so allowing them to download the files without restrictions. I can pay in the region of $30 for this project which must be completed within 5 days otherwise it will be cancelled. When bidding please mention if you have experience with joomla coding and docman component in particular. Thank you.