This post is for writers who like to use a non-proportional font like Courier in their hard copy manuscripts. The problem with Courier is that it doesn't print dark enough, but there's a solution. Change to Dark Courier, an HP font that's free to download.
If you use an Apple product, Courier Bold is the choice for you, but it's not free. You can get it at Fonts.com for $29.00. Just go to the site and do a search for that font name.
If you use a PC, try this link for Dark Courier download with instructions.
Another great site for PC fonts that's free is dafont. Great website. I use it a lot.
Dark Courier is not that much different from Courier Bold nor from Courier with bold face selected. The Courier font is designed to look what came from a typewriter, and it is mono spacing rather than proportional.
There are minor differences, but the amount of space taken up by a character in any of the 3 above is probably the same and will look the same to everyone but graphic designers who are font experts.
Having said all this about fonts, let me add: No editor is going to reject you because you used a proportional font instead of a mono spacing font.