Sum using this expression - but i need it to zero out per NEW ticket

[Tiger1]

I created a table with a column that should be getting the sum (total) of each particular field based on the new entry.
However, it is adding the total up in my table and using that total again even though i am generating a new ticket.

I used this expression:

SUM(airfare table[airfare amount])

Airfare Table - my table holding all airfare logs
Airfare Amount - amount for each individual airfare spent

I need this NOT to just keep adding them up. I want it to add up ALL airfare totals for that ONE ticket (expense request key)…

Thanks