At the bigger PostFinance, at least the ones in Basel (Freiestrasse, and Claraplatz), but presumably elsewhere too, they have a self-service machine you can dump the loose coins in to, and it gives you the receipt which you then take to the desk. No need to sort out the coins yourselves. It spits anything it doesn't like (eg Euro coins) back out to you. There wasn't any charge for doing it.

Once you give the counter staff the receipt, they credited my PostFinance account, but I think you also will give you the cash in notes if you ask them.