Extremely disappointed!

I went to spec-saver for a new pair of glasses, and there was no problem. When I went to collect they told me about an outstanding amount of 20% of the initial amount due to my medical aid which could only cover 80%. They did not let me know about this amount up until I went to collect, which I feel they should have send me a sms of given me a phone call but I did not receive any means of communication. I did not have the amount needed on the spot to cover the cost. About 2 weeks ago I went back and asked for a second pair which they had no problem helping me with as my medical aid covered the costs completely, today I received an sms the the pair is ready to collect and when I got there the refused to give me the second pair because of the unpaid amount on the first pair, but the second pair is completely paid yet I can't get the paid pair?!?!