They must have had a lot of money with them to buy all those groceries and have them delivered.
I assumed they took the money from George's wallet or found a cash stash in the house. This wasn't their first time playing the game.
No time for the old in-n-out, love, I've just come to read the meter.

There was no sign of forced entry. Remember, George used his key to get in the front, and they had to let the delivery boy in the back. Maybe they found a key before they left and knew it was a key to the house. They probably had enough money with them to buy a ticket for a bus right back to his house (or within walking distance to it). He stopped to make a phone call, stopped at the supermarket, so he didn't get back as fast as they did. They must have had a lot of money with them to buy all those groceries and have them delivered.

You have to assume they broke in.
I'd be more concerned with the little plot contrivance that they were able to beat him back to the house after they dropped him off.