When you want to buy another pot in your garden, do you have to use decorated eggs from your inventory, or do I have to click on the use button and send the eggs to the festival shop? Also, when you click use on the decorated eggs, is there a way to get them back to your inventory?

To get the pots, you use eggs from your inventory. Note the pots are one time use only.
Once you've used from the inventory, you can't get them back there.

ok thanks