Gamer has been removed
The trick with the Paper Route side quests is to STAY ON YOUR BIKE the entire time. You'll get the hang of it in no time at all.
The mistake people make is that they actually get off their bike and throw the papers at/in the mailboxes, and then try to run back and get on their bike to go to the next mailbox. Do not do this. Stay an your bike, and the achievement becomes easy.
Also, the 1st and the last paper route run are the only ones where you have to get 100%, the others do not require you to do them all.
Overall, this is a 2/10 difficulty achievement, so no worries. I't cake once you get the controls down.