Test of Between-Subjects Effects
The addition of Store ID as a random-effects factor reduces the overall unexplained variability, marked as Error, from 1522377.826 to 1073908.578. It has also reduced the variation explained by the main effects, USECOUP and SHOPFOR. This is quite natural and simply shows that some of the variation originally explained by these model terms can be better explained by Store ID.
The important fact here is that the proportional reduction in the variation explained by each of the main effects was not as great as the reduction in the unexplained variation. Thus, the partial eta squared has increased for each of the model effects.