It might not be a very well-known fact but Doug really liked shopping. Not just for video games or comic books, but also for clothes and the necessary beauty products (he remembered now that he was running short on moisturizer again.)
So Betsy's suggestion just made him smile instead of being appalled. He liked Betsy so how could he say no to spending time with her and shopping at the same time? He nodded and said, "Oh, sure! We should totally go shopping. If you don't mind me being kinda a picky shopper. I'm not that bad, I promise!"