问为什么我总是在python中得到一个未定义的变量错误消息？

EN

price = float(input("Please enter the price of the package: "))
distance = float(input("Please enter the distance of delivery in kms: "))

air = 0.36 
freight = 0.25

travel = input("Would you like to send your package via air or freight? ")
if travel == 'air':
    cost_travel = air * distance
elif travel == 'frieght':
    cost_travel = freight * distance
else:
    print("Error. Enter either air or frieght.")

full_insurance = 50
lim_insurance = 25

insurance = input("Would you like full insurance or partial insurance?")
if insurance == 'full insurance':
    cost_insurance = full_insurance
elif insurance == 'partial insurance':
    cost_insurance = lim_insurance
else:
    print("Error. Either enter full insurance or partial insurance.")

inc_gift = 15
no_gift = 0

gift = input("Would you like to include gift wrapping?")
if gift == 'yes':
    cost_gift = inc_gift
elif gift == 'no':
    cost_gift = no_gift
else:
    print("Either enter yes or no.")

priority_delivery = 100
standard_delivery = 20

delivery = input("Would you like priority or standard delivery?")
if delivery == 'priority':
    cost_delivery == priority_delivery
elif delivery == 'standard':
    cost_delivery == standard_delivery


total_cost = cost_travel + cost_insurance + cost_gift + cost_delivery
print(total_cost)