What do you do if you posted something but cane remember if you put a stamp on it?

[NCIRL]

Like the idiot I am. I received something I had to send back. They enclosed an envelope to use. I read the address but didn't actually check if they'd sent me a pre-paid envelope or not. I just posted it, and now feel like a right idiot.

Beyond standing at the letterbox from 5:50 until they empty it and asking if I can check (would they even let me?) there's nothing I can do is there?

I once did that. I put a note and a stamp in a plastic bag and stuck it to the outside of the post box. The lovely postie sorted it for me. We live in a quiet little hamlet though and know the postie well. I'm not sure you could do that in a city.

They'll just charge whoever you sent it to a fee.