Is it possible to upgrade the SSD on A1708 to 2TB?

While the SSD is a separate item on the 13" 2016-2017 non-touchbar model, there are no third party options as it is a new proprietary connector from Apple. An independent AASP may be able to purchase a larger SSD from Apple and install it for you, but the SSD would be extremely expensive and could approach or even exceed the cost of the laptop.

When it comes to the current Apple products, when you make your initial computer purchase make sure you purchase a large enough SSD and enough memory to last the life of the computer since most Apple computers are no longer upgradable since these parts are now integrated directly into the Logic Board.

That looks up to 2016 or 2017 MacBook Pro 13-in no touch-bar.

On those models the components of the SSD are solder to the mainboard for improved reliability. No internal field upgrades are possible.