I offer a few different tiers (all annual memberships) and often have members upgrade from a lower tier to a higher tier through the Portal. When they do so, they select the new tier and it charges them a difference between the price of the highest tier minus their current tier (and is also adjusted for how many days remain on their initial membership year).
This math or total is never shown to the customer, though. They don’t actually SEE what the charge will be on their card before they confirm and are charged.
Would love for them to be able to see that in the Portal UI!