How to calculate the average and max/min of an array of calculations from input?

I am trying to make a program that will read in items and its prices, and then produce the output reporting customers with the most and least money spent, as well as the average money they all spent and the corresponding customers' names.

I am having trouble calculating the accurate max/min/average. I suspect my error is when I calculate the total cost of all items purchased by customer (as stated in my comments).

I realize a hashmap, or using classes, may be easier but I have not learned it yet and our instructor does not allow for it so I am struggling with the technologies that I do know how to use. (for loops and arrays in this case)

This is what I have below:

import java.util.Scanner;
public class Store {
public static void main(String[] args) {
Scanner scan = new Scanner(System.in);
// Integer count of number of items in the store
int count = scan.nextInt();
// Create an array to store names and prices of each item
String[] itemName = new String[count];
double[] itemPrice = new double[count];
for (int i=0; i<count; i++) {
// Scan name of each item and price
itemName[i] = scan.next();
itemPrice[i] = scan.nextDouble();
}
// Integer count for the number of customers
int numCustomers = scan.nextInt();
String[] fName = new String[numCustomers];
String[] lName = new String[numCustomers];
double[] costs = new double[numCustomers];
double sum = 0;
double average = 0;
for (int j=0; j<numCustomers; j++) {
fName[j] = scan.next();
lName[j] = scan.next();
//Number of items bought
int numItems = scan.nextInt();
for (int k=0; k<numItems; k++) {
// For each number of items bought, name and quantity
int numItemBought = scan.nextInt();
String nameOfItem = scan.next();
// Find prices of the items customer bought
for (int i=0; i<numItems; i++) {
if (nameOfItem.equals(itemName[i])) {
double price = itemPrice[i];
// Calculate the total cost of all items purchased by customer
costs[i] = price * numItemBought;
}
}
}
}
// Calculate the sum and average
for (int l=0; l<costs.length; l++) {
sum = sum + costs[l];
average = sum/costs.length;
}
// Find the largest and smallest numbers
double temp;
for (int p=0; p<costs.length; p++) {
for (int j=p+1; j<costs.length; j++) {
if(costs
>costs[j]) {
temp = costs
;
costs
 = costs[j];
costs[j] = temp;
}
}
}
double smallest = costs[0];
double biggest = costs[costs.length-1];
System.out.println("Biggest: " + biggest + "\nSmallest: " + smallest + "\nAverage: " + average);
}
}

Example input:
6

Apple 0.25

Banana 0.75

Milk 3.15

Orange 1.25

Salami 2.50

Sponge 1.15

3

Carrie Brownstein 3 2 Banana 1 Orange 2 Milk

Corin Tucker 2 3 Banana 2 Sponge

Janet Weiss 1 5 Salami

Output:
Biggest: 6.3

Smallest: 0.0

Average: 2.85

When it should be

Biggest: Janet Weiss (12.50)

Smallest: Corin Tucker (4.55)

Average: 8.70

Problem #1: You fail to find prices for many items because the loop to find the price has incorrect bound:

                // Find prices of the items customer bought
for (int i=0; i<numItems; i++) {

Should be:

                // Find prices of the items customer bought
for (int i=0; i<count; i++) {

Problem #2: costs array keeps only the cost for the last item, instead of adding up the total cost, and also the cost is stored under the index of item, instead of the index of the customer:

                        // Calculate the total cost of all items purchased by customer
costs[i] = price * numItemBought;

To add up all the costs for the customer:

                        costs[j] = costs[j] + price * numItemBought;

It would help to rename the indexing variables so that they make sense: for example customerIndex instead of j and itemIndex instead of i

I think there's a lot to fix in your code.
For example, in this block:

// Find prices of the items customer bought
for (int i=0; i<numItems; i++) {
if (nameOfItem.equals(itemName[i])) {
double price = itemPrice[i];
// Calculate the total cost of all items purchased by customer
costs[i] = price * numItemBought;
}
}

Why do you use numItems, which is the number of item bought by 1 customer, to loop other all your catalog. If I buy 1 product, you will only do 1 iteration in your loop and only check the 1st product in your itemName array (so most likely you will not find my product and therefore don't update cost.
You should have something like this:

for (int i=0; i<itemName.length; i++) {
if (nameOfItem.equals(itemName[i])) {
double price = itemPrice[i];
// Calculate the total cost of all items purchased by customer
costs[i] = price * numItemBought;
}
}

An other issue (2 in fact) on this line:

costs[i] = price * numItemBought;

• everytime it's executed, you will overwrite the previously calculated price with a new one. But I would say what you want is to have the cost of all the items for each customer in cost. So you should do something like costs[i] = costs[i] + (price * numItemBought);

• And you're not using the correct index for your costsarray. It's define as an array containing the cost for each customer. So your index should represent a customer.
  But you're using i which is an index of product (as you can see because you're doing itemName[i]. So you're mistaking one thing for another and mix your data.
  In your current code, it's j which represent customer, so if you combine my 2 remarks, your code should be costs[j] = costs[j] + (price * numItemBought);

And there's surely others things to fix, but it's a first step.

My main advice would be to try to understand what exactly represent each variable you create. And don't hesitate to give them more meaningful name if it's easier for you. For example by replacing jby customerIndex, i by itemBoughtIndex and k by itemIndex. You can go back to the classical 'one letter naming' once you have more experience with loop and programming.