I feel like I may be stupid for asking...but I've been playing ACNL for 2 years and haven't seen this happen until today:

Ribbot pinged me and asked to buy my emperor butterfly off me for 2,500 bells. I agreed and then he said something along the lines of "Great! Here's your $3,000 bells!" and he gave me 3,000 bells instead of the original offer of 2,500.

Has this happened to anyone else, where a villager will offer you a certain amount of money for something but then pay you more??

Sounds like a glitch. I've been playing NL for 4 years now and never encountered that 500 bell tip for delivery.

lol, I'll take it!

I never sell anything to anyone because I don't want them junking up their houses with fossils, fish, and bugs. But it's great you got bonus bells!

You were wearing the lucky item of the day and your fortune was wealth/money.

The first time it happened to me I thought the villager made a mistake. The Jock villagers are the only ones that have given me extra bells.  It does say that you can get more money from rocks and from trees.

Oooohh interesting! I had no idea that could happen as a result of good money luck! Good to know!

I've had this happen several times, and it was always with a jock. That it doesn't happen with other personality types tells me this is definitely a bug. Another jock-specific bug is they'll sometimes ask for a small piece of furniture when they really want a rectangular one. That one drives me up the wall.

Sometimes villagers will give you all the money they have on them, which can be a lot or incredibly low. This recently happened with me and Cashmere, who offered 8,100 bells for a banded dragonfly, nearly 2x the normal sell price. I don't know for sure, but this may be affected by good money luck.

That's so weird! I've never heard of personality-specific bugs before...very cool

The bell one is a bug in programming. They neglected to match the bell values for the offer and trade dialogues. When this happens, jocks always give 20% more than they initially offered, so hey, you get a nice bonus!

The furniture one is an error in translation, but is still considered a bug in that it affects gameplay. Or it could be an error in communication between whoever writes the scripts and whoever is programming. The writer thinks it should be small furniture, but the programmer thinks it should be rectangular, and bang, the player gets annoyed.