What shops should I upgrade first?
With balanced demand amongst all shops, some shops run out of product before others because of inventory quantities. I see that I can pay to upgrade inventory, but I don't know whether I get more bang for my buck starting with shops that have the largest inventories already, or starting with the smallest, or if there are other factors besides inventory size to consider when upgrading. So which shops does it make best financial sense to upgrade first?
For upgrading, the increase in the default stock (75) is flat across the board.
This means that no matter the size of the store's stock quantities, the 'upgrade' is going to be the same constant amount.
However, if any workers there are working their 'Dream Jobs', the stock increase effectively doubles to 150 for the number of items matching Dream Jobbers.
As far as which stores to upgrade first, that depends on your playing preference. A general order follows:
If you're having trouble keeping stock in your smaller stores (especially your Food-based ones), then you can start with those to help balance things out. Having empty stores brings in nothing, while larger stores can be kept at least partially stocked without as much maintenance.
After that, prioritize upgrades to stores where the workers are in their 'Dream Job', as the additional stock bonus helps make the upgrade more worthwhile.
Otherwise, in general it's best to spread the upgrades around all of your floors so that they're consistently keeping their stock longer.
Last note, don't get too aggressive with your upgrades - unless you're frequently leaving your tower for long periods of time and have stores emptying out a lot, those Tower Bux can be better spent on other things like the Coin Bank and quick restocks/sales/builds.
Every shop upgrades the same way. From the Tiny Tower wiki:
At the cost of 3 Tower Bux, the player can permanently upgrade a commercial floor. Each level of upgrade does the following:
Adds 75 Stock to each product
Adds 45 Coins to the cost of restocking each product
Adds 5 minutes to the restocking time of each product