Australian Cafe charging 18% tax on Men?

This morning I was checking out my Twitter feed and I came across an article titled: "Australian cafe charges men 18% more to raise wage gap awareness" that was posted on Mashable. Being curious about this I clicked on the article and here is the basic gist of it:

Handsome Her, a vegan cafe in Victoria, Australia, has come up with a clever way to address the gender pay gap problem. The cafe put up a chalkboard outside their shop which explained that men will be charged an "18 percent premium to reflect the gender pay gap."

Additional rules include "women have priority seating" and "respect goes both ways".

The idea for the surcharge came from brainstorming ways to raise money for charity, which they plan to change every three months. But it's important to note, the "men tax" is completely voluntary. If a guy doesn't want to pay it, he's not forced to.

So since this tax is not mandatory what is the true purpose of this? Literally this accomplishes nothing for an issue that doesn't exist. Just seems like another pointless attempt by liberals to feel as if they are accomplishing something.