Since this is using geom_bar(), I believe there is a simple fix; in ggplot() set the reorder, ie;
:
ggplot (aes(reorder(new_cust, -total_sales),total_sales)) +
:
The -total_sales indicates we want the bars in descending order.
NOTE: You might need to set xlab() and ylab() to get your axis labels the way you want them
Something like: