Sending extra cards is a nice gesture that many people do when trading sports cards. The overwhelming majority of the trades I have personally made with people have included extra cards - close to, or even above 90% if I had to take a guess! So how do people know to include extras? I think it's all about the video maildays posted on Youtube. When you see people constantly trading and getting extras in their packages, I think you learn pretty quickly what the unwritten rule of including extras is.
So what kind of extras should you send? I always try to send extras that the other trader will like. To do this, you have to do a little digging on the person with whom you are trading. When dealing with Youtube trades, I usually take a quick glance at their main channel. If I see a certain team's logo scattered all over their page, it's usually a good indicator that I should find cards of that team to send off. Collectors also often put their favorite players in a list on their informational page or make a video telling others who they like to collect. Taking a minute or two to find this information will make the extra cards you send more meaningful to and appreciated by the person receiving them.
When dealing with trading on a forum, many users have a banner or information below the posts that they make. Taking a quick glance at that will yield the same result as above. It's not difficult to do at all, and well worth it to make the trade you participate in better. In the case where you can't find any information at all about the person you're trading with, you can always simply ask what player or team they like!
In terms of what to send, I usually like to send off extra base and insert cards. They are not a big loss to me, but may be really appreciated by the person I'm trading with. Also, I judge what extras to send with regards to the value of the deal we make. The higher and more value the deal, the better extras I send. For small trades, I just toss in a few inserts or a jersey card. Use your discretion, and just know that whatever is sent should be appreciated. It's really the gesture that counts!
Hopefully this article gave you some insight on sending extras. This practice is something that I think is really cool about the trading card hobby. It's all about giving more and trying to make another person's day a little better. It's totally awesome! I'd love to hear about any extras that you got that you really appreciated. Let me know about them in the comments section!