At my Airport we normally take all EX baggage at the ticket/sales desk as this is were we would keep thingss like Credit Card Machines, Sales Receipts and any Cash that is taken.
However I know that easyjet's check-in system is also their sales computer, so you can make bookings or change bookings at the check-in desk, but this is rare and we would only do this if flights are cancelled. I'm sure you could also take Excess baggage payments using this system aswell if you are paying by credit or debit card.
Other airlines could do this aswell if they do not have a ticket desk. It also helps to cut out the need for a ticket desk agent. This is good if the airline only operates 1 or 2 times a day.
Hope this helps.
Richard