Discount when paying early on Protected Content plugin

Hi guys,

I need to setup a 20% discount when the customer pays for a year in advance (the membership is usually paid monthly). Is there any way of doing this via the Protected Content Plugin, or do you have another plugin that complements it?

If not, I'll customize the plugin myself, so can you tell me what's the best way of doing that without breaking the "updatability" of the plugin? Maybe overrides in the child theme, or can you suggest what are the most likely hooks I should be looking at of the top of your head.

Many Thanks