Make sure you have enough caps (around 41500 with a barter skill at around 20, higher barter skill will give a lower price), or enough things to sell to get that many caps.
EDIT: the following method works on all commissary machines, credit to o Smeds o for that information.
On the High Road you will come across a sign which says "travel service", next to this sign is a truck with a commissary on the back, get ED-E to open it. Save the game (just in case), then sell your items until the commissary runs out of caps, it has 6000. Exit the buy/sell menu, then enter again, the commissary should now be back up to 6000 caps. Repeat until you have enough caps for all 3 of the Red Glare upgrades. Beware however, that the commissary will stop restocking caps if you buy anything from it.
Once you have enough caps purchase the 3 Red Glare upgrades, the achievement should pop when you exit the buy/sell menu.
You can now either continue with the upgraded Red Glare but with less caps, or you could reload the save and keep all your caps.