Eye drop past expiry date

I bought computer releif Similasan eye drops from Clicks on Sunday 12th July 2015 off the shelf and used it that same evening. I have used this product before so found it strange that I felt a burning sensation in my eyes. The next morning I asked my husband to put in the drops for me and he not needing reading glasses found underneath the box that it expired in May 2015. I called the branch on Monday to inform them and after speaking to 4 different people I was told to just bring it back and they will refund me. It was not about the money I said but the fact that they were selling expired items especially for your eyes. How dangerous is that and how do we know what else is expired on their shelves and her response was I should have checked the date before I used it but I wear reading glasses and didn't expect to purchase expired products from Clicks.what if it has done permanent damage ?