I think you can see how the function is derived from your data. Then if you want to know the probability of selling exactly 8 bags in 3 days, the steps are
1. Expand . This is easy if you have a computer algebra program, or you can just use Wolfram alpha.
2. Extract the coefficient of in the result. You will see in this case, that the coefficient is 0.12456, as you said.
If you would like to see the complete result of the expansion, go to Wolfram alpha
Wolfram|Alpha: Computational Knowledge Engine
expand (0.3 x^2 + 0.28 x^3 + 0.2 x^4 + 0.16 x^5 + 0.06 x^6)^3