Travel expenses - flat rates are being generated in TRIP

I have a scenario:

Employees either get 0.25 per kilometer with a maximum of 400Miles per trip OR

They can claim $55.00 per day if they travel in a Rental car

So I had configured 2 Trip types

a) Trip with own car (Miles)

b) Trip with rental car

and I had defined the Travel flat rates for Mileage & Rental for trip activity types, country groupings, reimbursement groupings: but the expenses are not being calculated....

Not sure what am I missing - can some one pls guide me?

Regards