store credit in positive balance but am told it has expired.

I was given a $50 iTunes gift card for Christmas 2015 - just on 4 months ago - and didn't use it until some time in January. There is $12.77 remaining.

A few minutes ago I tried to buy a $2.19 song but was told it had expired. Reading the very small print on the back of the card there seems to be a 2 year expiry date from date of purchase or last last activity on the redeeming iTunes account. My last purchase was on the 1st of April 2016.

Operating system is Yosemite 10.10.5 and iTunes is version 12.3.3.

Not sure how to contact Apple re this, as its not an problem with billing or purchase.

So any advice, insight or suggestions would be most helpful. While it's only a tiny amount of $$, it just seems wrong.

Thanks.