Alrighty, here's my final program:

Code:
#include <stdio.h>
#include <limits.h>

#define LOW 0.10151
#define MID 0.1411
#define HIGH 0.1680
#define TIER1 300
#define TIER2 500
#define MAX 100



int main ()
{
	int custid[30]; 
	int start[30];
	int stop[30]; 
	int remain[30];
	
	int counter;
	
	int i; 
	
	double energy;
	
	int low, mid, high, highest, lowest;
	
	char str[MAX];
	
 	double money;
  	double total;
	double amount[30];
	
	highest = INT_MIN;
  	lowest = INT_MAX;
  	low = mid = high = 0;
  	money = energy = 0;
 
   	for(i=0;i<MAX;i++)
            str[i]='\0';
 
   	printf("Enter a date [mm/dd/yyyy]: ");
   	scanf("%[^\n]s",str);
   	
   	printf("\nWhen finished, enter 0 to generate report.");
   	
   	i = -1;
   	
	while(custid[i++])
	{
		
		printf("\nEnter Customer ID #%d: ", i+1); 
		
		scanf("%d", &custid[i]);
		if (custid[i] == 0) break;
		printf("Enter start read for customer #%d: ", i+1);
		scanf("%d", &start[i]); 
		
		printf("Enter stop read for customer #%d: ", i+1);
		scanf("%d", &stop[i]);
	}
	
	counter = i; 
	
	for (i = 0; i < counter; i++)
	{	
		remain[i] = stop[i] - start[i];
	}
	
	for (i = 0; i < counter; i++)
	{
		if (remain[i] <= TIER1){ 
			amount[i] = remain[i] * LOW;
	 	}else if (remain[i] <= TIER2){ 
			amount[i] = remain[i] * MID;
		}else {
			amount[i] = remain[i] * HIGH;
		}
	}
  		
		  
	printf("***************************************************************\n\n");
	printf("Sparkie's Electric Utility Co. Billing report for %s\n\n", str);
	printf("Customer ID\tAmount Due\n");
  	
  	
  	
	for (i = 0; i < counter; i++)
	{
		printf("%d. %d\t $%.2f\n", i+1, custid[i], amount[i]);
	}
	
	for (i = 0; i < counter; i++)
	{
		if (remain[i] <= TIER1)
		{ 
			++low;		     					 
 		}
	 	else if (remain[i] <= TIER2)
	 	{ 
			++mid;				      				
 	 	}
	  	else 
	  	{
 	 		++high;
		}
	}
	
	for (i = 0; i < counter; i++)
	{
		if (highest < remain[i])     
      		highest = remain[i];	
		if (lowest > remain[i])
			lowest = remain[i]; 
	}
	
	for (i = 0; i < counter; i++)
	{
		energy += remain[i];
	}
	
	for (i = 0; i < counter; i++)
	{
		money += amount[i];
	}

	printf("Low Tier Users:  %d\t\tLow Tier = 0 to 300\n", low);
	printf("Med Tier Users:  %d\t\tMed Tier = 301 to 500\n", mid);
	printf("Hgh Tier Users:  %d\t\tHgh Tier = Over 501\n", high);
	printf("Average usage:   %.2f KWH\n", energy/(low+mid+high));
	printf("Highest usage:   %d KWH\n", highest);
	printf("Lowest usage:    %d KWH\n", lowest);
	printf("Total billing:   $%.2f\n\n", money);
	printf("***************************************************************\n");
	
	getch();	
}
I had to do away with my functions unfortunately in order to implement the new loop. I know I should be using functions, but for now I felt like this will suffice.

Really appreciate the help from everyone. I've learned more in two days than I have in the past six weeks.

Also, I'm sure my code is full of amateur-ism, so I'd still really appreciate any suggestions on what could be done differently/more efficiently :tntworth: