Can services be sold on Shopify with varying prices?

[TAEHOON]

Hi colleagues, I am trying to make a website that sells Korean revision service.

The service would receive a document (either .doc or .hwp) with the payment, and would return the document via the webpage 1-2 days later.

The web development would involve the following functions: 

1) varying prices per number of words (app. 1-3 cents per word), different prices for each customer

2) customers upload documents with the payment confirmation

3) seller posts the revised files (.doc or .hwp) on the website for the customers to see

I understand these functions are not native to Shopify, but I would like to assess if I can still construct the mentioned functions using Apps on Shopify.

If you can recommend me Apps, your advise would be much appreciated. Many thanks in advance.