Hopefully, it will have a nice big yellow sticker put on it for the full price of postage plus a £1 handling fee.
If you don't post it in time that's your fault and we charge for it, you have I think it is 3 days from purchase of online postage to post it. If you don't think you're going to post it in time get it refunded, buy new and in date postage and then send it.
It'll take longer to get there because it'll be surcharged, when it gets to the right region the person it is going to will get a nice card delivered saying they need to pay £x.yz before it is given to them. Once they've not paid for a few weeks if there is a return address it'll be sent back to your area, where you will need to pay the fee in order to get it returned to you. If there isn't a return address on it then consider it gone.
Of course you may have gotten lucky and it may make it through. Heed the lesson for next time though!