postcode uppercase vs lowercase

during checkout with credit/debit card on my site glendalefibres.com

if the customer enters there postcode in lower case it does not recognise it as valid

however it does work if the postcode is entered in uppercase

customers may see they have entered in their details correctly but abandon their cart because it says their postcode is invalid.

would any of these be possible?

A: fix it so it works with upper and lower case letter

B: make it so when the customer types is automatically in upper case

or

c: have a note in or around the postcode box telling the customer to use upper case