"Unexpected costs" refers to expenses that were not planned or budgeted for, catching someone by surprise. These are additional or unforeseen expenses that arise unexpectedly and can create financial strain or disruption. Full definition
A travel insurance plan can cover you for unexpected costs incurred before or during your trip. (travelinsurancecenter.com)
You'll experience unexpected costs of running a business around every corner. (fidelity.com)
I like the fact that I am responsible to pay for my dogs standard preventive care and the insurance only covers unexpected costs. (consumersadvocate.org)