First, you never change cancel variable so the condition is always True and it then loops forever. I assume you rather wanted to check full_name against q as per your input prompt.

Also thin_M = 'Thin Mints' assignment is pointless as it gets overwritten before any read, so just remove it.

And finally, 2nd while should rather be plain if.

full_name = input ('Enter a customers name (or q to quit): ')
address = input ("Enter the customer's address: ")
location = input("Enter customer's city, state, and zip code: " )
cancel = full_name.lower() == 'q'
while cancel == False:
  print ()
  thin_M = int(input('Enter the number of Thin Mints: '))
  if thin_M > 10:
    print ('Sorry we are accepting a maximum of 10 boxes per type.')
  else:
    cancel = True

You could use a boolean value to check if you are done. It will still iterate the rest of the loop but not execute the code. When it is done it will continue on its way without a break. Example Pseudo code below.

doneLogging = False
for i, x in enumerate(x):
    if not doneLogging:
        logging.info("Processing `x` n.%s...", i)
        y = do_something(x)
        if y == A:
            logging.info("Doing something else...")
            do_something_else(x)
        elif y == B:
            logging.info("Done.")
            doneLogging = True