Waiting outside is probably just dating manner.

Well, there are variations like having her wait while looking around the goods, but I think the message is, he will pay the bill so she doesn need to see how much or worry about paying him back. Asking her to wait make the position where he will carry the items for her, without needing to tell her he will do it for her.

It usually gives a smarter impression. With that situation (the woman waiting outside), the man didn need to ask, and the woman also doesn need to be in position where she asks him to do it for her (even if he is offering the service) or reject out of courtesy.

I understand your point, I just referred to it as being stupid to make her wait outside given the situation where she has the slave collar even more so after the situation that just happened recently in the guild, it's like asking to happen