Set A Mask For Zip Code In Checkout Page

Hey guys!

So basically, I got into another problem. I have field in Checkout form (Zipcode) and it is working fine, but I see 1 little problem here.

When customer goes through checkout, zipcode that he wrote goes to the invoice, which is fine, but not everyone will write zipcode as: LV-0000, this is zipcode in Latvia.

Now, problem is, some people can write zipcode as this -> 0000, which works too, but will not be understandable later, when receive invoice.

What should I do to put mask, so that, when user opens Checkout stage 2, and fills Zip code, he will see LV- in the text field, as a value.

It would be really easy to do in html: type="text" value="LV-", which would be clearly understandable, but I can't figure out how to do that.

So how would I do that?