Custom shipping is configured at the province level, not by town. You have three possible levels.
One cost for the whole country
You fill in only the Argentina field and it applies to every province equally.
One cost per province
You expand Argentina with the small arrow and enter each province separately.
Zones, for Mercado Libre Flex shipping
On top of provinces, Flex lets you configure the Greater Buenos Aires zones (cordones) separately.
Watch out here: the order in which Mercado Libre numbers the zones depends on where you or your logistics provider are located, so zone 1 doesn't always match the one you expect. If the amounts don't add up, try reversing the order, apply the changes to past data and check against a couple of real Flex orders.
If you have several zones within one province
Enter an average. Over a full period the result is very accurate, even if an individual order lands a bit above or below.
Other countries
Custom shipping is available for Argentina and Uruguay. For every other country, the shipping cost is taken directly from the order.