Does anyone know whether, if I post a letter today with an older stamp on it (with the value, not just "first class" on it), whether the Royal Maill will stick a "to pay" label on it because it won't be sorted until tomorrow? Or do I need to find a letterbox which gets collected today? Sending a birthday card to a friend overseas so it would be embarrassing if she had to pay to receive it!
I can't see on their website if there is any sort of grace period for things posted over the weekend.