A question about paying bills online?

I have a long past due auto loan bill from Capitol One of $566.48. When I was on the phone with them, the nasty lady (even though I was being nice and trying to resolve the problem) told me I had to pay immediately through Western Union or Money Gram. I got off the phone with her and went to Money Gram's website and they said I had to pick a location for the creditor to pick up there money and then I would have to call them again to give them the reference number. Well, I do not want to have to call those nasty people back again and again. If I go to my online account and make a payment using my checking account, how long will it typically take for them to receive the money. I don't want them repossessing my car so I'm hoping it would go through by today. Thanks for any help.