Post haste (after Colin Howard)

"I say, I'm pleased to see you," said the little man standing by the letter-box.
"Oh, hallo," I said, stopping. "Simpson, isn't it?"
The Simpsons were newcomers to the town, and my wife and I had only met them once or twice.
"Yes, that's right," answered Simpson.
"I wonder if you could lend me some money". I put my hand into my pocket. "You see," he continued, "my wife gave me a letter to post, and I've just noticed it isn't stamped. It must go tonight — it really must! And I don't think the post-office will be open at this time of night, do you?"
It was about eleven o'clock and I agreed that it wouldn't.
"I thought, you see, I'd get stamps out of the machine," explained Simpson, "only I find I have no small change about me."
"I'm sorry, but I'm afraid I haven't either," I said.
"Oh, dear, dear," he said.
"Maybe somebody else has," I said.
"There ins't anyone else".
We both looked up and down the street, but there was nobody to be seen.
"Yes, well," I said, intending to move off. But he looked so unhappy standing there with the blue unstamped envelope, that I really couldn't leave him alone.
"I'll tell you what," I said, "You'd better walk along with me to my place — it's only a few streets off — and I'll try to find some change for you there."
"It's really very good of you," said Simpson.
At home, we managed to find the money he needed. He thanked me and left. I watched him take several steps up the street and then return to me.
"I say, I'm sorry to trouble you again," he said. "The fact is we're still quite strangers round here and — well, I'm rather lost, to tell you the truth. Will you tell me the way to the post-office?"
I did my best. It took me several minutes to explain to him where the post-office was. At the end of that time I felt as lost as Simpson and decided to go along with him. I led the way to the post-office. Simpson put a penny into the automatic stamp-machine. The coin passed through the machine, but with no result.
"It's empty," I explained.
Simpson was so nervous that he dropped the letter on the ground and when he picked it up there was a large black spot on its face.
"Dear me," he said. "My wife told me to post the letter tonight. After all it's not so important but you don't know my wife. I had better post it now."
Suddenly I remembered that I had a book of stamps at home. "It will be posted," I said. "But we'd better hurry, or we'll miss the midnight collection."
It took rather a long time to find the book of stamps. But when we found it, we saw after all that it was empty. The fast thing I could advise him to do was to post the letter unstamped. "Let the other man pay double postage on it in the morning".
I took him firmly by the arm and accompanied him to the post-office in time for the midnight collection. He dropped in his letter, and then, to finish off my job, I took him home.
"I'm so grateful to you, really," he said when we reached his home. "That letter — it's only an invitation to dinner to Mr ... Dear me!".
"Why, what's the matter?"
"Nothing. Just something I've remembered."
"What?"
But he didn't tell me. He just opened his eyes and his mouth at me like a wounded goldfish, hurriedly said Good-night", and went inside.
All the way home I was wondering what it was he had remembered.
But I stopped wondering the next morning, when I had to pay the postman double postage for a blue envelope with a larpe black spot on its face.