I have created an email to merge the order product fields into it. A snippet of my Code is :

<tr>
                    <td style="background-color: #BA4697; color: #FFFFFF">
                        <b>Item/Package</b>
                    </td>
                    <td style="background-color: #BA4697; color: #FFFFFF">
                        <b>Amount</b>
                    </td>
                    <td style="background-color: #BA4697; color: #FFFFFF">
                        <b>Quantity</b>
                    </td>
                    <td style="background-color: #BA4697; color: #FFFFFF">
                        <b>Total</b>
                    </td>
                    
                </tr>
                       <apex:repeat value="{!relatedTo.OrderItems}" var="lineItem">
                        <tr>
                            <td>
                                 {!lineItem.PricebookEntry.Product2.Name}
                             </td>
                             <td>
                                {!lineItem.UnitPrice}
                            </td>
                             <td>
                                 {!lineItem.Quantity__c}
                            </td>
                            <td>
                                 {!lineItem.TotalPrice}
                            </td>
                        </tr>
                </apex:repeat>

I do get all the fields in order but do get an error for the Total price.
Error: Invalid field TotalPrice for SObject OrderItem

Any ideas?? ( the thing is that unit price and total price are under the object order item as I can understand. So its not a sum field that needs to sum up inside the email but pre-exists when you add the product and select the quantity and goes on the order products related list in Order)