Well interest on credit cards doesn't hit until a month after your next statement after the purchase. For instance my statement is on the 27th, so this won't be on my statement until Nov 27th, meaning I won't be charged interest if I pay it off in full by Dec 27th. I've been using my discover card for 10 years like this, always paying the balance off in full before the next statement. I average $300-400 a year in cashback bonus from using it and have never paid a dime in interest. They're great tools if you're disciplined.
Landed on exactly 500,000/500,000gs with exactly 18500 achievements and exactly 500 games. All 100% including DLC. Was going hard the first half of January to reach that goal! Finished it on 1-19-2016. Final achievement: Completion Serenity in Nero.