Part of my wp plugin is very simular to someone elses, copyright isseus?

I'm currently doing intern and I'm creating a webshop for one of our customers. We are using the WooCommerce plugin but there aren't any buttons to Continue shopping or Empty'ing your cart in one click.

So I decided to write an extension that gives you that and a couple of other features like adding a text that says ; "For $4.32 more you get shipping for free!". My plugin adds around 6 little features, but for the empty cart button I first checked another plugin on how to do it. That plugin has 20 php lines in total and I just don't know how to get the same result without making a lot of unnecessary changes and making the code longer.

The plugin happens to have license.txt with stuff about the GNU GENERAL PUBLIC LICENSE and that I may not make changes to their code. I don't want to publish my plugin and will only use it here on my intern but the webshop will off course go public. Is it okay for me to copy the actual 14 lines that I need, use my own variables or can't I do any of that ?

Not sure if I should've posted it here, but maybe my fellow programmers have encountered a similar problem like this before.

TLDR; Copied 14 lines of code with a GNU GP License, in my multi feature plugin. Am I allowed to do this ?

via Chebli Mohamed