"You are welcome." Lucifer has no intention of blocking his way to his purchase. After all, Lucifer had come here to do his own shopping which was interrupted by helping.

"I am happy to help whenever you might need." He's honest and then moves himself out of the way so that Matt can get past to the counter if he wants. Matt triggers Lucifer's desire to help because of his own son, though there is no way to know how the human will take the offer.