Writing a little plugin (special purchase)

-- HikaShop version -- : 2.6.1
-- Joomla version -- : 3.5

I am looking for somebody who can write a little plugin that does the following:

Whenever 3 products of a certain category are purchased the purchase price will be $99 instead of adding up the prices of the 3 products.

It should be a plugin that can be simply installed.

Hikashop support suggested to base it on

We are interested and available and I am sending you a Private Message with my questions.