She doesn't really need to wait, you know. Ryuuji's dept store doesn't just deal with slaves, they sell many things from food, high class wine to underpants. You can have her walk around browsing the store with her slave or something then comes back right in time when Ryuuji finalize his sale with Richelieu. She is going to host a party later that night, probably she can look at the wine section and select a crate to be sent to her boat, probably even get some food stuff too along the way. I'm not sure bout up there in Europe but in my country its quite common for high class dept stores to have items that the customer bought to be sent to their house for a rather small fee instead of giving the customer the trouble to lug around the items, especially if the items are big/heavy.