I am annoyed

With the postage rates rise at the end of March, I thought I had already memorised the prices and a few combinations to make up those rates. For some reason I thought First Class stamps were 64p! On some of the letters I posted last week and maybe the week before, I put 2 firsts and a 5p. 64p x 2 plus 5p would be £1.33. However, those first class stamps are only valued at 63p. So, I have put insufficient postage on for the airmail rate. The letters should still get to their destinations as the postage is more than sufficient to cover surface mail rate....