free memory used to return value in C

I have a function that calls another function which creates a unique ID and returns it as a string by malloc().

my question is how do I free the memory once outside the function? or what is the better way to do what I would like to do.

my function that asks for the ID:

void addCustomer(TennisStoreType* ts){

   CustomerNodePtr newcustdata;
   int datapos = 0;

   newcustdata = malloc(sizeof(CustomerNodeType));

   while (datapos <= CUSTDATA_POSITIONS) {

      char userinput[BUFFER_SIZE];

      switch (datapos) {
         case 0:
            strcpy(newcustdata->custID, createCustID(ts));
            break;
   }
}

the function that returns uniqueID:

char* createCustID(TennisStoreType* ts) {

   char *custID;

   custID = (char *)malloc(CUSTID_LEN + 1 * sizeof(char));

   if (ts->customerCount + 1 >= FOURDIGITS) {
      sprintf(custID, "C%i\n", ts->customerCount + 1);

   } else if (ts->customerCount + 1 >= THREEDIGITS){
      sprintf(custID, "C0%i\n", ts->customerCount + 1);

   } else if (ts->customerCount + 1 >= TWODIGITS) {
      sprintf(custID, "C00%i\n", ts->customerCount + 1);

   } else {
      sprintf(custID, "C000%i\n", ts->customerCount + 1);
   }

   return custID;
}

so how would I go about freeing the string that has been copied to newcustdata->custID? do I even need to? or is there a better way to go about doing this?

thanks in advance.