I just saw this under my earnings (next to the usual "sale" and "leads")...this one field just said "bonus" with a couple dollars next to it. It's for eBay.
You may have reached a performance bonus they have set up or they may ahve decided to bonus you for a sale you made that you werent credited enough for...
Usually, it'll say performance incentive.. on the performance bonus. My guess it's a tracking issue that they've corrected and credited you. They do a good job of catching tracking issues and 'making good'. hanji