I thought about making Lily's shop quest as an event but decided it was important to have a shop interface as a way to introduce the concept of shopping in the game. SRDude's Switch Shop plugin is pretty much using money to turn a switch on so it at least provides a "shopping" experience with the default interface. Since I need Yanfly's Shop Core plugin some of the stuff I plan on having in the game, I want to have the interface look the same between needing to buy regular items and special quest related items. Not just for this specific issue, I also really want a limited item stock type plugin so I can make the buy/sell economy a little more interesting but so far haven't been able to find a good one.
I appreciate the offer to help with the overworld but for the time being, it's one of the things I prefer to work on and figure out on my own. There's still way too many things I feel I need to have figured out before really diving into overworld stuff like the shape of the continents and where each realm is going to be place. etc.