Posted about letter without correct stamp on...

[DarlingDay]

Is my only option to wait by the post box for the postie to collect on Monday and stick another stamp in if he'll let me?

I thought a 1st class stamp was fine and it's an important document. Kicking myself.

I doubt whether a postie would want to go through all the post searching for your letter OP, perhaps better to contact the person the letter is being sent to, tell them you made a mistake with the postage you put on, and that if they have to pay to collect it, you'll reimburse them. That's what I'd do anyway.

It's not my letter it's posted on behalf of someone else and I don't think it had a return address on it. In a panic.

Don't worry about the lack of a return address - Royal Mail will still try to get it to the person its meant for. However, if they detect that you've underpaid, they will hold your letter at the delivery office and instead deliver a postcard asking the recipient to come to the delivery office and pay a fee, at which point they'll be given your letter.

Alternatively, if the postbox is a suburban one rather than a high street one, it may well be worth going to the postbox at first collection time (a lot of them just have one collection now) on Monday and asking the postie if you can stick another stamp on. There isn't usually much post in the suburban ones nowadays, so it shouldn't take you long to find your letter.

They aren't allowed to give you back the letter to do put another stamp on.

Does anyone know if it will just revert to 2nd class large letter, as a standard first class stamp costs more than a 2nd class large letter stamp?

I get legal correspondence as part of my job and when the stamps changed to the new QR one we do get the mail but often many months late.

As long as the value of the stamps is enough to cover the cost of it being delivered then you'll be fine. It'll go second class as technically you haven't underpaid.

It's definitely under 100g and the first class stamp is worth more than a large letter second class stamp.

What is making you think it's large letter size?

It's A4. I didn't realise large letter stamps existed. Until someone just told me!

I believe, according to Trudie at school, if you 'mess-up' your postage you have to send a letter to the Queen (it was some time ago that this conversation took place) with the stamp inverted (that means upside-down) with the word 'Sorry' written across it.
And then everything OK.