How to change scientific notation on legend labels in ggplot2

you can also use at the beginning of your code:

options(scipen=10000)

I figured it out. Basically all you have to do is insert the scales library and add labels = comma. Here's the modified code :

library(scales) 

ggplot(data = Canada2015_Import_3) +
  borders(database = "world", 
          colour = "grey60",
          fill="grey90") + 
  geom_polygon(aes(x=long, y=lat, group = group, fill = Trade_Value_mean),
               color = "grey60") +
  scale_fill_gradient(low = "blue", high = "red", name = "Trade Value", labels = comma) + 
  ggtitle("Canadien Imports in 2015") + 
  xlab("") + ylab("") + 
  theme(panel.background = element_blank(),
        plot.title = element_text(face = "bold"),
        axis.title.x=element_blank(),
        axis.text.x=element_blank(),
        axis.ticks.x=element_blank(),
        axis.title.y=element_blank(),
        axis.text.y=element_blank(),
        axis.ticks.y=element_blank())