Getting a Visualforce error on line 49 column 1
System.ListException: List index out of bounds: 0
Error is in expression '{!buy}' in component <apex:commandButton> in page confirmbuy: Class.StoreFront2.buy: line 49, column 1
Class.StoreFront2.buy: line 49, column 1

I'm trying to insert both a Purchase__c custom object record and a list of Merchandise__c line items at the same time. To do this I'm passing my "cart" values into a list and trying to add the merchanise items to the "counter" variable and insert them as "Merchandise__c" line items.
Apex

public virtual class StoreFront2 {    
    
    public String message { get; set; }
    List<DisplayMerchandise> products;
    Map<Id, DisplayMerchandise> cart;
    public Boolean incart{get;set;}
    public Id rowz;
    public id rowDel{ get;set; }
    public Boolean flip{ get;set; }
    public Boolean flag{ get;set; }
    public Boolean flag2 { get; set; }
    public Purchase_Line_Items__c ct{ get;set; }
    public Contact ct2{get;set;}
    
    public StoreFront2(){
        ct=new Purchase_Line_Items__c();
        ct2=new Contact();
        flag = false;
        flip = false;
        flag2 = true; 
      
    }
      
    public PageReference buy(){
        
        List<Merchandise__c> toMerch = new List<Merchandise__c>();
        
        List<id> updateMerch = new List<id>();
                
        PageReference send = new PageReference('/apex/StoreCart2');
        
        if(ct != null && cart !=null ){
            List<DisplayMerchandise> counter = new List<DisplayMerchandise>();
    		List<Merchandise_Line_Item__c> merchi = new List<Merchandise_Line_Item__c>();
            Decimal total = 0;
            counter = cart.values();
            system.debug(counter);
            
            
            for(Integer i = 0; i < counter.size(); i++){
                
                Decimal totalPrice = counter.get(i).count;
                
                total +=totalPrice;
                ct.Item_Quantity__c=total;     
                if(counter.size() > 0 || !counter.isEmpty()){
          		merchi.add(i,new Merchandise_Line_Item__c(Name =ct.Name,Purchases__c=ct.id,Merchandise_Item_Stable__c=counter.get(i).merchandise.id));
                }
            }	
            insert ct;
         	
            insert merchi;

      		
            
	
			
        }
        
        return send;     
    }
    
    
    public void doShow(){
        if(flip){
            flag = true;  
            flag2=false;
        }
        else{
            flag = false;
            flag2 = true;
        }
          
    }
 
    public PageReference shop(){
        handleTheBasket();
        message = 'You bought: ';
        for (DisplayMerchandise p:products){
            if(p.tempCount > 0){
               message += p.merchandise.name + ' (' + p.tempCount + ') ' ;
               }
            }
        return null;
    }
   
    
    public void remove(){
     rowz = (Id) ApexPages.currentPage().getParameters().get('rowDel');
        if(cart.containsKey(rowz)){
            cart.remove(rowz);
            if(cart.isEmpty()){
                incart = false;
            }      
        }  
       
    }
        public pageReference back(){
        PageReference doit = new PageReference('/apex/StoreCart');
        doit.setRedirect(false);
            return doit;
        }
    
    
    public Pagereference checkout(){
        if(cart.isEmpty()){
            ApexPages.Message myError = new ApexPages.Message(ApexPages.Severity.ERROR, 'Shopping Cart is Empty');
            ApexPages.addMessage(myError);
            return null;
        } 
        
        else{
        PageReference send = new PageReference('/apex/ConfirmBuy');
        return send;
        }
    } 
    
    
    public void handleTheBasket(){
        for(DisplayMerchandise c : products){
            if(c.tempCount > 0){
            if(cart.containsKey(c.merchandise.Id)){        
                cart.get(c.merchandise.Id).count += c.tempCount;
                
            }
            else{
                cart.put(c.merchandise.Id, c);
                cart.get(c.merchandise.Id).count = c.tempCount;
                incart = true;
      
            } 
        
        }
        }
        
    }
    
    public Map<Id, DisplayMerchandise> getCart() {
        if(cart == null){
            cart = new Map<Id, DisplayMerchandise>();
            incart = false;
                }
      
        return cart;
    }
    
    public class DisplayMerchandise {
        public Merchandise__c merchandise{get; set;}
        public Decimal count{get; set;}
        public Decimal tempCount{get;set;}
        public DisplayMerchandise(Merchandise__c item){
            this.merchandise = item;
            
        }
    }

    public List<DisplayMerchandise> getProducts() {
        if (products == null){
            products = new List<DisplayMerchandise>();
    
            for (Merchandise__c item :
            [SELECT id, name, description__c, price__c
            FROM Merchandise__c
            WHERE Total_Inventory__c > 0]) {
           
            products.add(new DisplayMerchandise(item));
            }
        }
        return products;
    }

}

Visualforce

<apex:page standardStylesheets="false" showHeader="false" sidebar="false" controller="StoreFront2">
    
    <apex:stylesheet value="{!URLFOR($Resource.styles)}"/>
    <h1>Store Front</h1>

    <apex:form >
        <apex:dataTable value="{!products}" var="pitem" rowClasses="odd,even">
            <apex:column headerValue="Product">
                <apex:outputText value="{!pitem.merchandise.name}" />
            </apex:column>
            <apex:column headervalue="Price">
                <apex:outputText value="{!pitem.merchandise.Price__c}" />
            </apex:column>
            <apex:column headerValue="#Items">
                <apex:inputText value="{!pitem.tempCount}"/>
            </apex:column>
        </apex:dataTable>

        <br/>

        <apex:commandButton action="{!shop}" value="Add to Cart" reRender="msg,cartPanel,cmdPanelId">
        </apex:commandButton>
    
        <apex:outputPanel id="msg">
            {!message}
        </apex:outputPanel>

        <br/>    

        <h1>Your Basket</h1>

        <apex:outputPanel id="cartPanel">
            <apex:dataTable id="cart" value="{!cart}" var="carti" rowClasses="odd,even">

                <apex:column headerValue="ID" rendered="false">
                    <apex:outputText value="{!cart[carti].merchandise.Id}" >
                    </apex:outputText>
                </apex:column>
                <apex:column headerValue="Product">
                    <apex:outputText value="{!cart[carti].merchandise.name}">
                    </apex:outputText>
                </apex:column>
                <apex:column headervalue="Price">
                    <apex:outputText value="{!cart[carti].merchandise.price__c}" />
                </apex:column>
                <apex:column headerValue="#Items">
                    <apex:outputText value="{!cart[carti].count}"/>
                </apex:column>
                <apex:column headerValue="Remove?">
                    <apex:commandButton action="{!Remove}" value="Remove" reRender="cart,cmdPanelId">
                        <apex:param name="rowDel" assignTo="{!rowDel}" value="{!carti}"/>
                    </apex:commandButton>
                </apex:column>             

            </apex:dataTable>
			<apex:outputPanel id="cmdPanelId">
				<apex:commandButton value="Checkout" action="{!checkout}" rendered="{!incart}" />
			</apex:outputPanel>
        </apex:outputPanel>

    </apex:form>

</apex:page>