Why did he say "can we get" instead of "can I get"? What is the difference in nuance?
"Can we get" is plural. In this context it implies that he wants to get something for him and others or just for others. If he were to say "can I get" in this case, this would imply that he wants something only for himself. However, since the store clerk wants to get paper bags for the three bears in this situation, he uses the plural form "can we get" to include the bears.