Use exceptions to catch user inputs that can not be summed up. Switch from testing any user input, to just guard the mathematical operation to sum up receipts.
total = 0
print("Welcome to the receipt program!")
while True:
add_to_total = raw_input("Enter the value for the seat ['q' to quit]: ")
if add_to_total == 'q':
break
try:
total += float(add_to_total)
except ValueError:
print "I'm sorry, but '%s' isn't valid. Please try again." % add_to_total
print("*****")
print "Total: $%s" % total