Posted letters with "old" stamps - what will happen?

[justdontcome]

When we sent out wedding invitations in January, several went to international addresses. Local post office sold us the £2.80 green stamps (or "slate blue" according to the RM website), the self-adhesive ones with a barcode.

Now that we're sending out thank-you letters, I had a few of these £2.80 ones left over and assumed I could use them again, so stuck them on and popped them in a postbox.

However, I just went to send another lot and found out at the post office that the price for international letters has gone up to £3.20 as of April this year.

Annoyingly, the first lot I sent was to the most "important" people - elderly relatives and those who had given us particularly generous presents - so I'm feeling really foolish and annoyed with myself.

Does anyone know what will happen? I'm aware that letters are sometimes delivered with a request to pay the missing postage, so I'm wondering if I should give these people a heads-up and apology in advance? Unless there's some kind of grace period?

Argh!!! Just realised I also posted a few to UK addresses with first class stamps I'd bought back in January - these were special edition ones and don't have a price written on them, just "1st" and "2nd" - does anyone know what will happen with these?

Christmas type/special edition ones are fine
No clue on the international issue tho

The ones with a price on will be considered to have insufficient postage, so either they'll be returned to you by the destination country's postal service (which will take ages, so maybe let those people know you have sent one!) or they'll be asked to pay a surcharge. The ones that just say 'first' or 'second' will still be valid.

@Destiny123 ah, great, thank you, so at least the UK ones are safe.

@BarnacleBeasley thank you, that's good to know - I'll give them a heads up in that case.

So annoyed that I used them specifically for my elderly relatives and our most generous guests; I wanted to get those out ASAP and now I've screwed it up... oh well... lesson learned...