Contents less inside than what is written on the outside packaging

On the 4th of November I bought 12 portions free range chicken from Woolworths Main Road Kenilworth ***. I didn't open the product until Sunday, the 7th when I was about to cook it. I found 11 pieces of chicken instead of 12 as written on the package. And how i noticed this was because I was having 4 guests and intended to give 2 pieces each. Since I didnt have time to go back to the store and return the chicken I went ahead with my cooking. Today i phoned the store in question to inform them. The lady I spoke to Nolene (if i'm not mistaken) told me I should have returned the chicken on the day so i could get a refund or exchange for another pack as they need proof. I told her it would have been inconvenience for me to do that as i was pressed for time to finish cooking before my guests arrived. I told her it is not about getting a refund, it is the principle that i paid for something and didn't get full value for it. This has never happened to me before and i am long time a customer to the woolworths store but this is not the kind of customer service i expected. Please check the contents inside the package is the same as what is written on the outside packaging before they are sent to the stores. I am not going to be counting pieces of chicken every time in the store. Thank you