Open question: Missing delivery period for my condition tax repayment. What now do I do?

I would send into my condition zustandrückerstattung on Monday, but I forgot to set a stamp on the envelope. It was taken never and my forgetful grandmother let it on the kitchen table. I only saw it straight today, therefore I did not have an idea, which was sent back it and I thought that it had gone through. Now 15 April past is there, can I someone a stamp on it straight set and him again send, or did have I call or a form, an etc. receive?