求救！收到了国税局的信。。

I got a letter from IRS in the mail. It claims that I filed late this year and therefore there is a penalty of about $300 I need to pay. It used the date that my check was cashed (5/11/06) as the file date. But in fact I did file on time (on 4/15/06) by certified mail. I kept the mail receipt initially. Then in May I saw my check was cashed and I think okay they got it so I discarded the receipt (what a stupid mistake!). Now what can I do? Can I challenge IRS by asking them to prove I filed late? Please help! Thanks a lot!!!