How Much Does It Cost To Upgrade MacBook Air 2019 13 inch

Apple only adds a larger SSD if you buy it when you purchase your laptop. They don't offer aftermarket upgrades.

My 2017 Air has a 256 gb SSD and I've added a 1 tb Samsung SSD for additional storage. It's connected to one of my Air's USB ports through an enclosure.

Nothing. They will not upgrade it for you. You can add additional storage externally. See OWC.

Just to add, if you’re purchasing a new one, it can be ordered with a larger built-in SSD. The SSD components are soldered directly into the logic board, and not into any kind of socket or replaceable connection.