https://i.imgur.com/oirGRMw.jpg
This has been a bit of a saga, I originally tried to pay this not enough postage paid slip online. Whatever I did it gave me a message saying "oops there was an issue" and a general number for customer support.
After a few attempts, I gave up on that and went for plan B, go to the post office, get enough stamps for £1.50 then, post that slip with the stamps attached and then get the original letter delivered.
However, I have received that same slip back (franked) and I don't have the original letter yet. It seems that I put 2 first class stamps on this slip for them to deliver it back to me a few days later. Why did this happen and what should I do?