Yes, I think he just pumped gas outside also. There would be no reason to go inside to pay. Since he was able to purchase gas with a debit card, he must have received the pin number from Natalee before he killed her. He wanted to test it and see if it was the correct pin number. At 4am, about the only place you could do that would be a gas station (maybe a 24 hour CVS), but a gas station is ideal because he was able to do it at the pump without entering the building. I wonder if he made any other purchases, either online or in person the following day?
I, also, think he paid outside and never went in. I use my debit card all the time and just choose "credit" and do not need a pin number.