I forgot to charge them AGAIN

The last time I worked for this family I didn't charge them for my taxi, so tonight I told them I was going to add the amount onto the bill. This time time I add on the last evening's taxi but not one for tonight - so am still out of pocket. It's £7 so I'm not exactly starving through missing the money (!) but the principle is there - I'm annoyed that I've left that money when I could have it! If you see a note blowing about the street to you watch it wash down the storm drain or do you pick it up, dry it out, and spend it. Of course - that's MINE!