Accounting Software Linked with API from PayPal / Skrill

In theory this is a simple task. We'd like to create a custom Accounting Software that collects all our sales data from Skrill and PayPal. We want the data from the API to be used to work out profit per sale. This data should then be used to work out daily, weekly, monthly and yearly profits. The API data from Skrill and PayPal provides the product name and the net amount we receive after payment gatewat fees. So the net price that we receive after PayPal & Skrill fees minus the cost per product will give us the profit figure per sale. Detailed Explanation: The system should work as follows: 1.) Customer makes purchase from website 2.) API sends data to Accounts database 3.) Account database sorts data and minuses the cost of products sold 4.) Profit per sale figure shown For the above to work there should be a section within the software where we add all our inventory and the cost of the products. The cost of stock changes daily / weekly. Therefore we need to able to add the cost of for example, 100 units of Xbox 1 game (cost price 2 GBP). The system should therefore work out profit based on the cost price of 2 GBP and only for 100 units at this price. The next day if we add the following: Xbox 1 game (cost price 3 GBP), the system should work out profit for the next 100 sales based on the new cost price. For now we want to keep it as simple as explained above. The software should however be expandable in the future. Before accepting any offers we will discuss the above with you to make sure you understand the project. Please read it, no time wasters.