How to plot choropleth map using zip codes?

Question

I want to plot a dataset into a chloropleth map using the zip codes. The score represents the density of the desired chloropleth map regions

  ZIP.CODE  score
1   10000   5.400       
2   10001   88.750      
3   10002   135.050     
4   10003   86.025      
5   10004   12.775      
6   10005   7.800   

I have tried to plot using ggmap but can't find a way to incorporate zip codes in it.
Thanks in advance.