"The box is closed."
Why is קופסא correct as well? Seems weird that you can just change a letter and it still be the same word.
There are some words that originate from Aramaic in which they used to end with an aleph (א'). Some examples: דוגמא, גרסא, טבלא, אסמכתא, משכנתא.
The Hebrew academy decided that the correct ending is ה' (except for a few exceptions) but some people are used to the old spelling and many still spell it like that, so I guess that's why both are accepted.