15xp is for a "near pin".
Basically, whenever a customer buys/sells something, they have a "perfect price" in mind they hope you'll give them. If you get that price exactly (or almost?), you'll get a "full pin", which gives you 30xp (instead of the basic 10). If you're very close to the price, you'll get a "near pin".
Not only do these give you more merchant xp, but they also raise your reputation with that customer faster than a normal sale.
Check out Doniazade's FAQ
for more info on this (and other useful stuff!).
And now for something unrelated:
Louie put in an Advance Order for two "metal items". I was ecstatic; great opportunity to equip him with two items of my choice!
Or so I thought...
I tried a few combinations (a sword + various pieces of armor), and he only ended up equipping one of the items, usually the sword. I didn't think of testing to see what makes them choose which item at the time.
A bit later, Charme put in an order for two pieces of armor, so I tested this further.
I suspected that she'd equipped the first item of the two I selected, but this wasn't the case.
I'm not entirely sure by what means they decide which of the items to actually equip, but my guess is that it follows the "Optimum Equipment" parameters.
Point being, though, don't try to multiple-equip one of your adventurers when fulfilling an advance order, as it won't work. Pick one item you want them to have, and best to give them something they -can't- equip for the other(s), to make sure the item you want is the one that gets equipped. No doubt giving them something equal to or weaker than what they already have or to what you're giving them to equip is a safe bet as a secondary (or tertiary, etc.) item, but why take chances?